8. If the equation $a(a-x)-b(b-x)=0$ has infinitely many solutions for $x$, then ( ).
(A) $a+b=0$
(B) $a-b=0$
(C) $\frac{a}{b}=0$
(D) $a b=0$
Answer: $\mathrm{B}$.
Solution: Organize the original equation, we get
$$
(a-b) x=a^{2}-b^{2} \text {. }
$$

Since
the equation has infinitely many solutions,

then
$$
a-b=0 \text { and } a^{2}-b^{2}=0 \text {. }
$$

When $a-b=0$, $a=b, a^{2}-b^{2}=0$, so when $a-b=0$, the original equation has infinitely many solutions, hence the choice is (B).