According to Hairston Et Al

According to Hairston et al.,why is the world green?

A) Hairston et al. speculated that herbivores overcome plant defenses, but ultimately plants have a faster growth rate and so dominate the world.
B) The assumption was that plants dominate the food web.
C) Hairston et al. thought that the world is green because of top-down control on herbivores.
D) The authors believed that the world is green because of bottom-up control on herbivores.