Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Example Input: Paragraph- Sent 1: Static electricity is like a teeter-totter.
Sent 2: What happens if a teeter-totter is balanced?
Sent 3: Correct!
Sent 4: It does not move.
Sent 5: What about when its not balanced?
Sent 6: Yes, it will now begin to move.
Sent 7: Charges move when they are not balanced.
Sent 8: Charges can build up by friction.
Sent 9: Maybe you rub your feet on a wool mat or carpet.
Sent 10: Rubber soled shoes readily gain charges.
Sent 11: The wool carpet easily gives up charges.
Sent 12: The two items become unbalanced.
Sent 13: One item has a positive charge.
Sent 14: The other has a negative charge.
Sent 15: The difference in charge is called static electricity.
Sent 16: Just like the teeter-totter, something is going to move.
Sent 17: Positive charges build up on the mat.
Sent 18: Negative charges build up on you. 
Question: What happens if static electricity is not balanced?.
Example Output: There would be a positve or negative charge.

Example Input: Paragraph- Sent 1: The Agencies Confer When they learned a second plane had struck the World Trade Center, nearly everyone in the White House told us, they immediately knew it was not an accident.
Sent 2: The Secret Service initiated a number of security enhancements around the White House complex.
Sent 3: The officials who issued these orders did not know that there were additional hijacked aircraft, or that one such aircraft was en route to Washington.
Sent 4: These measures were precautionary steps taken because of the strikes in New York.
Sent 5: The FAA and White House Teleconferences.
Sent 6: The FAA, the White House, and the Defense Department each initiated a multiagency teleconference before 9:30.
Sent 7: Because none of these teleconferences-at least before 10:00- included the right officials from both the FAA and Defense Department, none succeeded in meaningfully coordinating the military and FAA response to the hijackings.
Sent 8: At about 9:20, security personnel at FAA headquarters set up a hijacking teleconference with several agencies, including the Defense Department.
Sent 9: The NMCC officer who participated told us that the call was monitored only periodically because the information was sporadic, it was of little value, and there were other important tasks.
Sent 10: The FAA manager of the teleconference also remembered that the military participated only briefly before the Pentagon was hit.
Sent 11: Both individuals agreed that the teleconference played no role in coordinating a response to the attacks of 9/11.
Sent 12: Acting Deputy Administrator Belger was frustrated to learn later in the morning that the military had not been on the call.
Sent 13: At the White House, the video teleconference was conducted from the Situation Room by Richard Clarke, a special assistant to the president long involved in counterterrorism.
Sent 14: Logs indicate that it began at 9:25 and included the CIA; the FBI; the departments of State, Justice, and Defense; the FAA; and the White House shelter.
Sent 15: The FAA and CIA joined at 9:40.
Sent 16: The first topic addressed in the White House video teleconference-at about 9:40-was the physical security of the President, the White House, and federal agencies.
Sent 17: Immediately thereafter it was reported that a plane had hit the Pentagon.
Sent 18: We found no evidence that video teleconference participants had any prior information that American 77 had been hijacked and was heading directly toward Washington. 
Question: When did the Secret Service initiate security enhancements around the White House?.
Example Output: After the second plane hit the world trade towers.

Example Input: Paragraph- Sent 1: Hundreds of thousands of years before China was to become the world's longest-running civilization, the prologue was enacted by means of the flicker of a carefully tended fire.
Sent 2: Peking Man, a forebear of Homo sapiens, achieved a mastery of fire.
Sent 3: We might call it the first Chinese invention.
Sent 4: Not that he devised flint and steel, matches, or any other way of creating fire.
Sent 5: Peking Man simply learned how to capture flame, perhaps from a forest fire, and keep it alight.
Sent 6: He thus enjoyed two revolutionary luxuries: light and heat.
Sent 7: Technologically and sociologically, it was a phenomenal breakthrough: with fire, communities could live year 'round in one cave, in which cooking and even smelting could be pursued.
Sent 8: And so, by 600,000 b.c., about 50 km (31 miles) southwest of present-day Beijing, the ancestors of mankind were ready to settle down.
Sent 9: Several hundred thousand years later, when Marco Polo reached the capital of China, he was astonished by a further development in fire technology.
Sent 10: The Chinese, he announced, used black stones dug out of mountains as fuel.
Sent 11: Europeans did not yet have a word for "coal," nor had they discovered a use for it.
Sent 12: The First Dynasty The confluence of mythology and history in China took place around 4,000 years ago during what is referred to as the Xia (Hsia) Dynasty.
Sent 13: This was still the Stone Age, but the people are thought to have made silk from thread produced by the worms they cultivated on the leaves of their mulberry trees.
Sent 14: And written language (which evolved as early as 4,500 to 5,000 years ago) was already in use, originally by oracles and then by official scribes — China's first scholars.
Sent 15: During the second of the quasi-legendary dynasties, the Shang (from about the 16th to 11th centuries b.c.), the Chinese developed an interest in art.
Sent 16: Careful geometric designs as well as dragon and bird motifs adorned bowls and implements.
Sent 17: And with the arrival of the Bronze Age, the Chinese created bronze vessels of such beauty and originality that, until modern times, archaeologists refused to believe they were cast 3,000 years ago.
Sent 18: The Shang Dynasty gave rise to the concept of one Chinese nation under one government. 
Question: Which European explorer discovered that the Chinese used black stones dug out of mountains as fuel?.
Example Output:
Marco Polo.

