Given a scientific passage and an answer, generate a question for the given answer.

Example Input: Passage: Bond lengths are intermediate between covalent single bonds and covalent double bonds. Answer: covalent double bonds
Example Output: Bond lengths are intermediate between covalent single bonds and what?

Example Input: Passage: Improving crops by selecting for certain genetic traits. The desired traits promote productivity. Recently, genetically engineered crops have been introduced. Answer: promotes productivity
Example Output: What is the advantage of selecting for certain genetic traits for crops?

Example Input: Passage: Composite volcanoes usually have craters on the top. Why are the craters sometimes "U" or horseshoe-shaped?. Answer: composite
Example Output:
A crater can usually be found on the top of what kind of volcanoes?