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

[EX Q]: Passage: Population density is the average number of individuals in a population per unit of area or volume. For example, a population of 100 insects that live in an area of 100 square meters has a density of 1 insect per square meter. If the same population lives in an area of only 1 square meter, what is its density? Which population is more crowded? How might crowding affect the health of a population?. Answer: population density
[EX A]: What is the term for the average number of individuals in a population per unit of area or volume?

[EX Q]: Passage: Volatile organic compounds (VOCs) are carbon compounds such as methane. VOCs are released in many human activities, such as raising livestock. Livestock wastes produce a lot of methane. Answer: methane
[EX A]: The waste of cows releases a lot of which type of gas?

[EX Q]: Passage: The Krebs cycle is the second stage of cellular respiration. Answer: krebs cycle
[EX A]:
What cycle is the second stage of cellular respiration?