Given a scientific passage and an answer, generate a question for the given answer.
Example: Passage: A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:. Answer: nucleotides.
Example solution: A frameshift mutation is a deletion or insertion of one or more of what that changes the reading frame of the base sequence?
Example explanation: The science passage defines frame mutation and the main compoment here is nucleotides. Hence the generated question is apt.

Problem: Passage: Fossil fuels are compounds of carbon and hydrogen, called hydrocarbons . Hydrocarbons can be solid, liquid, or gas. The solid form is coal. The liquid form is petroleum, or crude oil. The gaseous form is natural gas. Answer: coal
Solution: What do we call the solid form of hydrocarbons?