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

Ex Input:
Passage: Simple carbohydrates can be classified in terms of chain length. Answer: simple

Ex Output:
Which carbohydrates can be classified in terms of chain length?

Ex Input:
Passage: Heat waves have increased in frequency and duration in recent years. The summer 2011 North American heat wave brought record temperatures across the Midwestern and Eastern United States. Many states and localities broke records for temperature and for the most days above 100 o F. Answer: cell membrane

Ex Output:
What are all cells surrounded by?

Ex Input:
Passage: Vaccines are available for some infectious diseases. For example, there are vaccines to prevent measles, mumps, whooping cough, and chicken pox. These vaccines are recommended for infants and young children. Answer: vaccines

Ex Output:
What is used to prevent some infectious diseases?