Which Statement Regarding Biological Clocks Is True

Which statement regarding biological clocks is true?

A) The period of an animal's free-running rhythm is not affected by light‒dark cycles.
B) Biological clocks are very sensitive to internal (cellular) temperature.
C) Nocturnal animals often have periods of free-running rhythms much shorter than 24 hours.
D) Biological clocks permit the timing of processes during 24-hour periods when environmental cues are unreliable.