Here's a problem to solve: 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?

Among the 4 following options, which is the correct answer?

- A: altitude above sea level
 
- B: types of local plant cover
 
- C: circulation patterns of regional air masses
 
- D: distance to the nearest mountain range
 