The Genetic Basis for the Circadian Cycle Involves a Loop

The genetic basis for the circadian cycle involves a loop of gene activity: the Clock/Bmal1 complex turns on the activity of both per and cry genes; products from these bind with a protein product of the tau gene; the per/cry/tau complex suppresses the action of the Clock/Bmal1 complex, etc. This cycle takes about _____ hour(s) to complete.

A) 1
B) 4
C) 12
D) 24