What Environmental Change Likely Influenced the Evolution of Bipedalism in Humans

What environmental change likely influenced the evolution of bipedalism in humans?

A) Trees grew taller, making it necessary to reach higher to pick fruit.
B) Warming of the Earth made it an advantage to get the heat-sensitive brain higher above a hot substrate.
C) Evolution of fleeter predators, making it necessary to use the hands to fight them off, rather than try to outrun them.
D) The Earth cooled and there was a resultant change from a forested to a grassy landscape.
E) Evolution of longer hind limbs made quadrupedal locomotion less effective.