Given the task definition and input, reply with output. In this task, you are given a sentence and a question, you would be asked to create the answer which is contained in the sentence provided.

Sentence: 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. Question: Which is the best explanation for how air masses move across the United States?
The prevailing westerlies move air masses from west to east across the United States but may be deflected by the jet stream.