Cocklebur Will Not Flower If Exposed to 8 Hours of Dark

Cocklebur will not flower if exposed to 8 hours of dark and 16 hours of light.It will 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 cocklebur will not flower.What triggers cocklebur to flower?

A) short dark periods
B) short light periods
C) long light periods
D) long dark periods
E) No answer is correct.