I Take Two Pictures of My Dog Kaycee

Multiple Choice

I take two pictures of my dog Kaycee.I use the same film and level of illumination for both pictures,but I halve the f-number for the second picture.By what factor will the required time of exposure change if the film is to receive the same total light energy?

A) 4.0
B) 0.50
C) 2.0
D) 0.25