A short-day plant will flower only when A) days are shorter than ...

[xclash]

A short-day plant will flower only when
A) days are shorter than nights.
B) days are shorter than a certain critical value.
C) nights are shorter than a certain critical value.
D) nights are longer than a certain critical value.
E) days and nights are of equal length.

[clippers!]

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