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

[EX Q]: The mesopause is at the top of the mesosphere.
[EX A]: What is at the top of the mesosphere?

[EX Q]: The long, threadlike extensions of the nerve cell help transmit nerve impulses.
[EX A]: Which part of the nerve cell helps transmit nerve impulses?

[EX Q]: Human blood has a buffering system to minimize extreme changes in ph.
[EX A]:
Human blood has a buffering system to minimize extreme changes in what?