Glycolysis is called an anaerobic pathway for ATP synthesis because

a. O2 is not a substrate in any of the reactions
b. it occurs only during intense (anaerobic) activity (activity that is so short that we don't need to move O2 to the muscle)
c. it occurs when the muscle fiber is anaerobic -- there is no O2 in the cell
d. it occurs only during easy (anaerobic) activity (activity that is slow enough that we don't need to move O2 to the muscle)