Chrysanthemums and Other Short-Day Plants Flower Only When the ____

Chrysanthemums and other short-day plants flower only when the ____.

A) hours of darkness are greater than a critical value
B) hours of darkness fall below a critical value
C) light exposure is constant for a few days
D) temperatures fall below a critical value for an extended period of time
E) temperatures are above a critical value for an extended period of time