If a short-day plant has a critical night length of 15 hours, ...

[xclash]

If a short-day plant has a critical night length of 15 hours, then which of the following 24-hour cycles will prevent flowering?
A) 8 hours light/16 hours dark
B) 4 hours light/20 hours dark
C) 6 hours light/2 hours dark/light flash/16 hours dark
D) 8 hours light/8 hours dark/light flash/8 hours dark
E) 2 hours light/20 hours dark/2 hours light

[clippers!]

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