Gross Primary Production Is Higher in Tropical Forests Than Boreal

Gross primary production is higher in tropical forests than boreal forests primarily as a consequence of what ecosystem characteristic?

A) Greater canopy layering in the tropics relative to the boreal zone increases leaf area index.
B) Most trees in the tropics are broadleaf while boreal trees primarily have needle leaves.
C) There is more available soil water in tropical forests than in boreal forests.
D) Photosynthetic enzymes are more efficient in hot tropical climates than in cool, boreal climates.