Interrupting the Night with a Flash of Light Can Trigger

Interrupting the night with a flash of light can trigger flowering in a long-day plant.Why doesn't introducing a short period of darkness during the day cause a short-day plant to flower?

A) Sunlight contains enough far-red light to inhibit phytochrome induction of flowering.
B) A brief dark period is not sufficient to trigger flowering.
C) Phytochrome is only active in the dark.
D) The circadian clock is less responsive during the day than at night.