TASK DEFINITION: Given a scientific passage and an answer, generate a question for the given answer.
PROBLEM: Passage: Biofuels are useful because they are liquid. Biofuels can go into a gas tank unlike many other types of alternative energy. Answer: liquid

SOLUTION: What form do biofuels take that makes them very useful?

PROBLEM: Passage: The amount of material left over after a certain number of half-lives can be easily calculated. Answer: pheromone

SOLUTION: What term is used to describe a chemical released by an animal that affects the behavior or physiology of animals of the same species?

PROBLEM: Passage: When waves of two different frequencies interfere, a phenomenon known as beating occurs. The frequency of a beat is the difference of the two frequencies. Answer: beating

SOLUTION:
When waves of two different frequencies interfere, what phenomenon occurs?