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

Example Input: Passage: An experiment is a controlled scientific study of specific variables. A variable is a factor that can take on different values. For example, the speed of an object down a ramp might be one variable, and the steepness of the ramp might be another. Answer: variable
Example Output: In a controlled scientific study, what do you call a factor that can take on different values?

Example Input: Passage: 16.3 Circulatory and Respiratory Systems Animal respiratory systems are designed to facilitate gas exchange. In mammals, air is warmed and humidified in the nasal cavity. Air then travels down the pharynx and larynx, through the trachea, and into the lungs. In the lungs, air passes. Answer: nasal cavity
Example Output: In mammals, air is warmed and humidified in what?

Example Input: Passage: Protists do not create food sources only for sea-dwelling organisms. For instance, certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute an essential step in the digestion of cellulose ingested by these insects as they bore through wood. Answer: cellulose
Example Output:
Certain anaerobic parabasalid species exist in the digestive tracts of termites and wood-eating cockroaches, where they contribute an essential step in the digestion of this?