altitude above sea level types of local plant cover circulation patterns of regional air masses distance to the nearest mountain range
I gave my students this multiple choice question: Going north from the equator, hot, moist climates are replaced by hot, dry climates. The boundary between these two climate types is determined by which factor?

Only one answer is correct among these 4 choices:
- altitude above sea level
- types of local plant cover
- circulation patterns of regional air masses
- distance to the nearest mountain range

Could you tell me which one is correct?

circulation patterns of regional air masses