Detailed Instructions: 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 (", ").
Q: 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:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
1, 3.