Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

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
What do we call the solid form of hydrocarbons?