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

[Q]: Passage: When there is no mass in a volume of space, the space is not curved. An object passing through such space would follow a straight line in our normal way of thinking of a straight line. Answer: curved
[A]: When there is no mass in a volume of space, the space is not what?

[Q]: Passage: Hydraulic car lifts also use fluid to transmit pressure and increase force. The lifts are used to raise cars, which are very heavy, so mechanics can work on them from underneath. Controls in airplanes use fluids to transmit pressure and increase force so a flick of a switch can raise or lower heavy landing gear. To see animations of hydraulic systems such as these, go to URLs:. Answer: hydraulic systems
[A]: What kind of systems use fluid to transmit pressure and increase force?

[Q]: 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
[A]:
What do we call the solid form of hydrocarbons?