C R Bainton Conducted an Experiment in Which He Increased the the Workload

C. R. Bainton conducted an experiment in which he increased the workload of dogs by having them run on inclined treadmills and showed that respiratory rate responded to CO₂ levels. However, this success story followed an earlier experiment that did not support his hypothesis. In the earlier study, he

A) compared respiratory rate of dogs versus cats.
B) compared sleeping versus barking dogs.
C) changed the speed at which the dogs ran.
D) changed the dog's motivation by keeping them food-stressed.
E) injected dissolved O₂ into the dogs' arteries.