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

Let me give you an example: Earth rotates on its axis once times in one day.
The answer to this example can be: How many times does Earth rotate on its axis in one day?
Here is why: The given output correctly poses a question based on the scientific fact given.

OK. solve this:
The best explanation for how air masses move across the united states is that the prevailing westerlies move air masses from west to east across the united states but may be deflected by the jet stream.
Answer:
Which is the best explanation for how air masses move across the United States?