Zach undergoes a sperm analysis and is upset to learn that he has oligospermia.This means that

A) he has a sexually transmitted disease.
B) he has cancer of the spermatogonia, and his sperm will never mature.
C) he makes too many sperm, and they will crowd and kill each other.
D) he makes too few sperm, lowering the probability of one sperm successfully reaching and fertilizing an oocyte.
E) his sperm have two heads, each of which contains an entire genome of DNA, making them too heavy to swim to an oocyte.