The sum of coefficients of variable terms in 9-x-y is:
1
-1
0
9
Given expression 9-x-y=9-x+y
The coefficient of variables are
xis-1yis1
Sum of coefficient of variables =-1+1=0
on adding these two we get 0.
Thus, the correct option is C.
Like terms are the terms that contain same variables, same exponents of variables and same coefficients.