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

Q: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only contract.

A: Skeletal muscles must work in pairs to move bones back and forth at a joint because they can only do what?
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Q: A diet rich in calcium and vitamin d may reduce the risk of osteoporosis and related bone fractures.

A: A diet rich in calcium and what vitamin may reduce the risk of osteoporosis and related bone fractures?
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Q: Do particles collide more in two reactants fluid they are both in fluid forms or solid forms.

A:
Do particles collide more in two reactants when they are both in fluid forms or solid forms?
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