You are given a paragraph, a question and some answer options (associated with "A", "B", "C", "D", etc.) You need to list all the correct answer options with their associated letters (e.g. "A" can be a valid answer). Note that sometimes, more than one option can answer the question correctly and completely. In those cases, please generate all such options separated by a comma (e.g. "A, B" is a valid answer). Do not generate anything else apart from one or more of the following characters: 'A', 'B, 'C', 'D', etc.

Let me give you an example: Paragraph- Sent 1: Obama was born on August 4, 1961, at Kapiʻolani Maternity & Gynecological Hospital in Honolulu, Hawaii.
Sent 2: He is the only President to have been born in Hawaii.
Sent 3: He was born to a white mother and a black father.
Sent 4: His mother, Ann Dunham (1942-1995), was born in Wichita, Kansas, of mostly English descent, with some German, Irish, Scottish, Swiss, and Welsh ancestry. Question: How old was Obama's mother when he was born? (A) teenager (B) in his 40s (C) mid 20s (D) almost twenty.
The answer to this example can be: D.
Here is why: Obama was born in 1961. His mother was born in 1942 and 1961-1942=19. Therefore, "almost twenty" answers the question sufficiently.

OK. solve this:
Paragraph- Sent 1: Imagine you are standing in a farm field in central Illinois.
Sent 2: The land is so flat you can see for miles and miles.
Sent 3: On a clear day, you might see a grain silo 20 miles away.
Sent 4: You might think to yourself, it sure is flat around here.
Sent 5: If you drive one hundred miles to the south, the landscape changes.
Sent 6: In southern Illinois, there are rolling hills.
Sent 7: Why do you think this is?
Sent 8: What could have caused these features?
Sent 9: There are no big rivers that may have eroded and deposited this material.
Sent 10: The ground is capable of supporting grass and trees, so wind erosion would not explain it.
Sent 11: To answer the question, you need to go back 12,000 years.
Sent 12: Around 12,000 years ago, a giant ice sheet covered much of the Midwest United States.
Sent 13: Springfield, Illinois, was covered by over a mile of ice.
Sent 14: Its hard to imagine a mile thick sheet of ice.
Sent 15: The massive ice sheet, called a glacier, caused the features on the land you see today.
Sent 16: Where did glaciers go?
Sent 17: Where can you see them today?
Sent 18: Glaciers are masses of flowing ice. 
Question: In what part of Illinois might you be able to see a grain silo that is 20 miles away? (A) Central Illinois (B) Springfield (C) Southern Illinois (D) Northern Illinois.
Answer:
A.