If gcd(a,b)=12\gcd(a, b) = 12gcd(a,b)=12, which of the following expressions MUST be divisible by 12?
3a+7b3a + 7b3a+7b
2a−5b2a - 5b2a−5b
a+b+5a + b + 5a+b+5
ababab