Select the irreversible reactions of glycolysis.
a. conversion of glucose 6-phosphate to fructose 6-phosphate by phosphoglucose isomerase
b. conversion of glucose to glucose 6-phosphate by hexokinase
c. conversion of fructose 6-phosphate to fructose 1,6-bisphosphate by phosphofructokinase
d. conversion of phosphoglycerate to phosphoenolpyruvate by enolase
e. conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase