Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

[EX Q]: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only contract.
[EX A]: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?

[EX Q]: The male reproductive system consists of structures that produce sperm and secrete testosterone.
[EX A]: What does the male reproductive system consist of?

[EX Q]: Muscarinic receptors can cause both depolarization or hyperpolarization depending on the subtype.
[EX A]:
Muscarinic receptors can cause both depolarization or hyperpolarization depending on the what?