A is the average (arithmetic mean) of the first 7 positive multiples of 5 and B is the median of the first 3 positive multiples of positive integer n. If the value of A^2 – B^2 is zero, what is the value of n?	Choose the correct choice:
A) 5  B) 6  C) 10  D) 15  E) 20

Rationale: "if a ^ 2 - b ^ 2 = 0 , then let ' s assume that a = b . a must equal the 4 th positive multiple of 4 , thus a = 20 , which also equals b . b is the second positive multiple of n , thus n = 20 / 2 = 10 . the answer is c ."
Answer: C