- Prevailing westerlies are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude
- United states are located in the middle latitudes between 30 and 60 degrees latitude
- So, prevailing westerlies are prevailing winds that move from west to east across the united states

- Prevailing westerlies are prevailing winds that move from west to east across the united states
- Wind can make air masses to move in the same direction
- So, the prevailing westerlies move air masses from west to east across the united states

- The prevailing westerlies move air masses from west to east across the united states
- Jet streams may deflect the air masses moved by the prevailing westerlies
- Therefore, the prevailing westerlies move air masses from west to east accross the united states but may be deflected by the jet streams

So, the prevailing westerlies move air masses from west to east accross the united states but may be deflected by the jet streams
Question: Which is the best explanation for how air masses move across the United States?
Context: - Wind can make air masses to move in the same direction 
- Jet streams may deflect the air masses moved by the prevailing westerlies 
- Prevailing westerlies are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude 
- United states are located in the middle latitudes between 30 and 60 degrees latitude