ATP is a co-substrate of the enzyme phosphofructokinase (PFK/PFK-1). In most species, ATP is also an inhibitor of PFK-1 at higher concentrations. Which statement below would provide a suitable explanation for this discrepancy?

A. There are two sites on PFK-1 that bind ATP. One is the active site; the other is the regulatory site where inhibition occurs.
B There must be another cofactor interacting with ATP at high concentrations to achieve inhibition of PFK-1.
C. ATP is unable to bind to the allosteric site on PFK at high concentrations.
D. ATP actually activates the reverse of the reaction preceding the PFK-1 step in the pathway. It likely has no direct effect on PFK-1.