A region of the canary forebrain shrinks during the nonbreeding season and enlarges when breeding season begins. This change is probably associated with the annual

A) addition of new syllables to a canary's song repertoire.
B) crystallization of subsong into adult songs.
C) sensitive period in which canary parents imprint on new offspring.
D) renewal of mating and nest-building behaviors.
E) elimination of the memorized template for songs sung the previous year.