If a and b are positive integers, and a=5b+20, the greatest common divisor of a and b CANNOT be  Choose the correct choice from the following options.
A) 2  B) 4  C) 5  D) 6  E) 10

Rationale: if b is 2 , 4 , 5 , or 10 , then gcd of a and b is 2 , 4 , 5 , and 10 respectively . so , by poe the answer must be d . still : if b is a multiple of 6 , then a is 20 greater than a multiple of 6 , so not a multiple of 6 , so both of them can not be divisive by 6 . answer : d .
Answer: D