The Ability of Plants to Measure Night Length Was Determined

The ability of plants to measure night length was determined by experiments in which

A) plants were grown in 12 hours of darkness alternating with 12 hours of light.
B) plants were grown in continuous light.
C) plants were grown in dark periods that were interrupted with brief pulses of light.
D) flowering was measured in plants of different ages that had been placed on the same light-dark schedule.
E) plants were grown in a 24-hour cycle in which the length of the light period was increased gradually.