You are given a paragraph, with numbered sentences, and a question. Write the numbers of the sentences needed to answer the given question. Separate multiple sentence numbers with a comma (", "). E.g. "1, 2" would be the correct response if Sent 1 and Sent 2 are needed to answer the given question (do not include sentences that contain no information that is necessary to answer the question). Sort the sentence numbers in an increasing order (i.e., '1, 2', instead of '2, 1'). Your answer should not contain anything than the sentence numbers separated by a comma (e.g. '1, 2, 3, 4'). The given question requires more than one sentence to answer. So, your answer must contain at least one comma (", ").
Example: Paragraph- Sent 1: A Republican bid to keep the government open past Friday includes no new money for the construction of a border wall along the U.S.-Mexico border, according to several congressional aides familiar with ongoing talks.
Sent 2: GOP leaders submitted the new offer Tuesday afternoon in an effort to appease Democrats, whose votes are needed to avert a shutdown of federal agencies, several House and Senate aides said.
Sent 3: However, Republicans also insisted on increases in border security and defense spending, including money to repair existing fencing and new surveillance technology to patrol the nearly 2,000-mile border.
Sent 4: Democrats have indicated that they would support such appropriations so long as no money goes toward an actual wall.
Question: Who has to be appeased to keep the government open?.
Example solution: 1, 2.
Example explanation: The question is: Who has to be appeased to keep the government open? The phrase "appease Democrats" is mentioned in Sent 2, about accepting offer that is mentioned in Sent 1 to "keep the government open". So, Sent 1 and Sent 2 are the sentences needed to answer this question.

Problem: Paragraph- Sent 1: Tuesday, September 11, 2001, dawned temperate and nearly cloudless in the eastern United States.
Sent 2: Millions of men and women readied themselves for work.
Sent 3: Some made their way to the Twin Towers, the signature structures of the World Trade Center complex in New York City.
Sent 4: Others went to Arlington, Virginia, to the Pentagon.
Sent 5: Across the Potomac River, the United States Congress was back in session.
Sent 6: At the other end of Pennsylvania Avenue, people began to line up for a White House tour.
Sent 7: In Sarasota, Florida, President George W. Bush went for an early morning run.
Sent 8: For those heading to an airport, weather conditions could not have been better for a safe and pleasant journey.
Sent 9: Among the travelers were Mohamed Atta and Abdul Aziz al Omari, who arrived at the airport in Portland, Maine.
Sent 10: Boston: American 11 and United 175.
Sent 11: Atta and Omari boarded a 6:00 A.M. flight from Portland to Boston's Logan International Airport.
Sent 12: When he checked in for his flight to Boston, Atta was selected by a computerized prescreening system known as CAPPS (Computer Assisted Passenger Prescreening System), created to identify passengers who should be subject to special security measures.
Sent 13: Under security rules in place at the time, the only consequence of Atta's selection by CAPPS was that his checked bags were held off the plane until it was confirmed that he had boarded the aircraft.
Sent 14: This did not hinder Atta's plans.
Sent 15: Atta and Omari arrived in Boston at 6:45.
Sent 16: Seven minutes later, Atta apparently took a call from Marwan al Shehhi, a longtime colleague who was at another terminal at Logan Airport.
Sent 17: They spoke for three minutes.
Sent 18: It would be their final conversation. 
Question: Who was Atta's final conversation with ?.

Solution: 16.