Which of the following explanations accounts for heliotropism in sunflowers?
a. The east and west sides of the stems elongate at different rates during the day and night
b. The plant is just turning towards the light (phototropism) and the clock is not involved
c. There is an organ in the plant that is circadian regulated and swings the plant from side to side
d. The circadian clock genes are expressed differently on the east and west sides of the stem