$1 \cdot 16$ If $x=\frac{a}{b}, a \neq b$ and $b \neq 0$, then $\frac{a+b}{a-b}$ equals
(A) $\frac{x}{x+1}$.
(B) $\frac{x+1}{x-1}$.
(C) 1 .
(D) $x-\frac{1}{x}$.
(E) $x+\frac{1}{x}$.
(42nd American High School Mathematics Examination, 1991)
[Solution] $\frac{a+b}{a-b}=\frac{\frac{a}{b}+1}{\frac{a}{b}-1}=\frac{x+1}{x-1}(\because b \neq 0)$. Therefore, the correct choice is $(B)$.