Why is the head of a sperm covered with a sac of enzymes?

A)The enzymes are used to prevent other sperm from entering the egg after the sperm and egg membranes have fused.
B)The enzymes are needed to digest a hole in the protective "shell" surrounding the egg.
C)The enzymes are released to kill other sperm so that, by the time sperm reach the egg, only a hundred or so remain to compete with.
D)The enzymes are gradually released into the sperm, where they break down sugars, releasing the energy needed to power sperm movement.