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

Input: Consider Input: 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

Output: What is the term for the average number of individuals in a population per unit of area or volume?

Input: Consider Input: Passage: Dissolved mineral salts wash into the ocean. As ocean water evaporates, it leaves the salts behind. This makes the water saltier. Ocean water is about 3.5 percent salts. The main salt is sodium chloride. Answer: salts

Output: Concentration of what, the substance left behind when ocean water evaporates, is about 3.5 percent?

Input: Consider Input: Passage: If the experimental value is less than the accepted value, the error is negative. If the experimental value is larger than the accepted value, the error is positive. Often, error is reported as the absolute value of the difference in order to avoid the confusion of a negative error. The percent error is the absolute value of the error divided by the accepted value and multiplied by 100%. Answer: negative
Output: If the experimental value is less than the accepted value, the error is what?