Which of following CANNOT be the greastest common advisor of two positive integers x and y？

a: 1

b: x

c:y

d:x-y

e:x+y

e

x+y>x 且 x+y>y,

一个数的最大公约数不大于该数

严谨一点的话，X+Y>=X and X+Y>=Y

而等号只能在出现0的时候成立.

Thanks so much!!! I mistook the meaning of divisor, 他也可以做公约数讲，除了除数的意思外。