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

Example Input: Passage: Descriptive statistics and graphs help organize data so patterns and trends are more apparent. Descriptive statistics include the mean and range. Types of graphs include bar, circle, and line graphs. Answer: graphs
Example Output: Bar, circle, and line are all types of what?

Example Input: Passage: Pea plants are naturally self-pollinating. In self-pollination , pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand with pollen from other parent plants of his choice. When pollen from one plant fertilizes another plant of the same species, it is called cross-pollination . The offspring that result from such a cross are called hybrids. Answer: cross-pollination
Example Output: What is it called when pollen from one plant fertilizes another plant of the same species?

Example Input: Passage: 6. Mercury is small, rocky and covered with craters. Why?. Answer: craters
Example Output:
Mercury is small, rocky and covered with what objects?