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

[EX Q]: Very low density is the concentration of gas molecules in the mesosphere.
[EX A]: What is the concentration of gas molecules in the mesosphere?

[EX Q]: Wilting occurs when water lost by transpiration is not replaced by absorbtion from room.
[EX A]: What occurs when water lost by transpiration is not replaced by absorbtion from room?

[EX Q]: There are four classes of biochemical compounds.
[EX A]:
How many classes of biochemical compounds are there?