1. Given two monomials about $x, y$: $8 x^{3-6} y^{2 b-3}$ and $-a x^{3-2 a} y^{b-a}$, their difference is still a monomial. Then the value of $a+b$ is ( ).
(A) 2
(B) 3
(C) $\pm 2$
(D) 2 or 3
-1. D.
(1) When $a \neq 0$, the two monomials should be like terms, thus:
$$
\left\{\begin{array} { l } 
{ 3 - b = 3 - 2 a , } \\
{ 2 b - 3 = b - a }
\end{array} \Rightarrow \left\{\begin{array}{l}
a=1, \\
b=2
\end{array} \Rightarrow a+b=3\right.\right. \text {. }
$$
(2) When $a=0$, the difference of the two monomials is the monomial $8 x^{3-6} y^{2 b-3}$. According to the definition of a monomial, $3-b$ and $2 b-3$ must both be positive integers. Thus, $b=2$.
$$
\text { Hence } a+b=2 \text {. }
$$