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.
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.
Example solution: D.
Example explanation: Obama was born in 1961. His mother was born in 1942 and 1961-1942=19. Therefore, "almost twenty" answers the question sufficiently.

Problem: 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: Why does Springfield have its features? (A) Wind erosion (B) The ground is capable of supporting grass and trees, so wind erosion would not explain it (C) Tornado erosion (D) River erosion (E) A long time ago, it was covered by a mile thick sheet of ice (F) The massive ice sheet, called a glacier, caused the features on the land (G) A glacier.

Solution: E, F, G.