If an integer n is to be selected at random from 1 to 400, inclusive, what is probability n(n+1) will be divisible by 8?
Options
A) 1 / 4  B) 1 / 8  C) 1 / 2  D) 2 / 3  E) 3 / 4

Rationale: "for n ( n + 1 ) to be a multiple of 8 , either n or n + 1 has to be a multiple of 8 . thus n must be of the form 8 k or 8 k - 1 . the probability is 2 / 8 = 1 / 4 . the answer is a ."
Answer: A