A tissue culture of vertebrate muscle was provided with a constant
excess supply of glucose under anaerobic conditions starting at time
zero and the amounts of pyruvic acid and ATP produced weremeasured. The solid line in the graph aboverepresents the pyruvic acid
produced in moles per liter per minute. ATP levels were also found to
be highest at points A and C, lowest at B and D. A second culture was set up under the same conditions, except that substance X was added, andthe results are indicated by the dotted line

The rate of pyruvic acid formation fluctuates because
(A) all glucose has reacted.
(B)all enzymes have been used up.
(C) the reaction is accelerated by positive feedback.
(D) the reaction is affected by negative feedback.
(E)coenzymes have begun to function.