The Reason That Treelines at the Same Latitudes North and South

The reason that treelines at the same latitudes north and south of the Equator occur at different altitudes is .

A) that there are few mountains in the Southern Hemisphere
B) the Sun is usually higher in the sky in the Northern Hemisphere
C) unknown
D) that there is more ocean in the Southern Hemisphere
E) the varying solar radiation received by each hemisphere