Read the following excerpt from shakespeare's "sonnet 73" and answer the question.

that time of year thou mayst in me behold when yellow leaves, or none, or few do hang upon those boughs which shake against the cold, bare ruined choirs, where late the sweet birds sang.

why have the birds figuratively stopped singing?

a)because they were too late.
b)because it is too hot.
c)because the choir is gone.
d)because it is winter.