Clover Will Flower If Exposed to 8 Hours of Dark

Clover will flower if exposed to 8 hours of dark and 16 hours of light.It will not flower if exposed to 16 hours of dark and 8 hours of light.However,if the 16 hours of dark is interrupted with a brief period of light the clover will flower.Which triggers clover to flower?

A) short dark periods
B) long dark periods
C) short light periods
D) long light periods
E) both long and short dark periods