TASK DEFINITION: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
PROBLEM: A penguin must do increase muscle mass before molting.

SOLUTION: A penguin must do what before molting?

PROBLEM: Zinc is active transition metal kind of metal.

SOLUTION: Zinc is what kind of metal.?

PROBLEM: 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.

SOLUTION:
Which is the best explanation for how air masses move across the United States?