Refer to the figure.
These results, showing the change in testis length in male red crossbills over time, support the hypothesis that crossbills

A) are reproductively opportunistic birds that maintain full reproductive capacity throughout the year.
B) have an underlying reproductive cycle that is dependent on photoperiod.
C) only use seed abundance, an unpredictable environmental cue, to determine reproductive capacity.
D) do not rely on environmental cues to entrain their reproductive cycle.