For a group of n people, k of whom are of the same sex, the (n-k)/n expression yields an index for a certain phenomenon in group dynamics for members of that sex. For a group that consists of 20 people, 5 of whom are females, by how much does the index for the females exceed the index for the males in the group?
Options:
A) 0.05 B) 0.0625 C) 0.5 D) 0.25 E) 0.6

Rationale: "index for females = ( 20 - 5 ) / 20 = 3 / 4 = 0.75 index for males = ( 20 - 15 / 20 = 1 / 4 = 0.25 index for females exceeds males by 0.75 - 0.25 = 0.5 answer : c"
Answer: C