Greatest Common Factor??...

A question says:

what is the grestest common factor of the positive integers, J and K?

1. K=J+1

2. JK is divisible by 5.

Answer is A.

I am confused, for the statement 1, only 1 can be the common factor? but can 1 be the grestest common factor between any two numbers?....

就是1啊，为什么不可以呢？