A long-day plant will flower if A) the duration of continuous ...

[xclash]

A long-day plant will flower if
A) the duration of continuous light exceeds a critical length.
B) the duration of continuous light is less than a critical length.
C) the duration of continuous darkness exceeds a critical length.
D) the duration of continuous darkness is less than a critical length.

[clippers!]

I chose D and got the answer, hope that helps.