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

Q: Saturated hydrocarbons hydrocarbons contain only single bonds between carbon atoms.

A: What hydrocarbons contain only single bonds between carbon atoms?
****
Q: Axillary buds in the axils of leaves and stems give rise to branches.

A: What in the axils of leaves and stems give rise to branches?
****
Q: The main function of the circulatory system is to transport materials.

A:
What is the main function of the circulatory system?
****