A biologist measures the allele frequencies of pea plants in a very controlled environment. The plants can either have a dominant tall allele (T) or a recessive short allele (t). Which of the following is a reason that this population is not at Hardy-Weinberg equilibrium?

A. The pea plants are mated at random.
B. Both alleles ensure equal survival.
C. One pea plant mutates to have a new allele.
D. Every pea plant reproduces exactly once.