Who challenged the ideas of Hairston et al.and what did they contend?

A) The hypothesis by Hairston et al. was never challenged because it has been empirically demonstrated that the world is green due to top-down control.
B) Murdoch challenged the ideas of Hairston et al. and argued that the world is green because herbivores are limited by the defenses plants have evolved.
C) Pimm in 1966 argued that the world is green because herbivores are overwhelmed by their predators.
D) Hanski and Levins aregued that the world may still be green even if it is controlled from the bottom up.