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

Example input: Earth rotates on its axis once times in one day.
Example output: How many times does Earth rotate on its axis in one day?
Example explanation: The given output correctly poses a question based on the scientific fact given.
Q: The term concentration describes the number of items in a given area or volume.
A:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
What term describes the number of items in a given area or volume?