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

[EX Q]: Three many families are known quarks divided into.
[EX A]: How many families are known quarks divided into?

[EX Q]: Saturated hydrocarbons hydrocarbons contain only single bonds between carbon atoms.
[EX A]: What hydrocarbons contain only single bonds between carbon atoms?

[EX Q]: Of course, a net external force is needed to cause any acceleration, just as newton proposed in his second law of motion.
[EX A]:
Of course, a net external force is needed to cause any acceleration, just as newton proposed in his second law of what?