If a and b are positive integers, and a=3b+12, the greatest common divisor of a and b CANNOT be	Pick one:
A) 1
B) 3
C) 2
D) 5
E) 4
If b is 1, 2, 3, or 4, then GCD of a and b is 1, 2, 3, and 4 respectively. So, by POE the answer must be D.   Still: if b is a multiple of 5, then a is 12 greater than a multiple of 5, so not a multiple of 5, so both of them cannot be divisive by 5.   Answer: 5.
<result>D</result>
If a and b are positive integers, and a=3b+12, the greatest common divisor of a and b CANNOT be