The Pigment Protein Phytochrome Is Thought to Allow Plants to Detect

The pigment protein phytochrome is thought to allow plants to detect photoperiod because

A) it can be converted into two different forms depending on the specific wavelengths of light that it absorbs.
B) it absorbs red light during the day and breaks down in darkness.
C) the photoperiod response depends on how much light is absorbed by phytochrome.
D) the pigment becomes inactive in the dark.
E) the pigment response can be observed only in intact living plants.