Which of the Following Is True as Exercise Intensity Increases

Which of the following is true as exercise intensity increases?

A) At a certain point aerobic metabolism is shut off and anaerobic metabolism is turned on.
B) When the anaerobic threshold is reached muscles stop getting oxygen and suddenly begin producing lactate, which can be measured in the blood.
C) There is a point in ultraendurance activities where fat metabolism stops completely and protein metabolism supplies nearly all of the energy.
D) Energy is supplied in a smooth continuum where one system may predominate depending upon the intensity.