Why Is the Head of a Sperm Covered with a Sac

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.