ACT Math Practice Question: Simplifying algebraic fractions

Try this ACT math practice question on simplifying and rewriting algebraic fractions.:

If $a$, $b$, and $a-\dfrac{1}{b}$ are not $0$, then
$$ \dfrac{\dfrac{1}{a}-b}{a-\dfrac{1}{b}} $$ equals

1. $\quad -\dfrac{a}{b}$
2. $\quad -\dfrac{b}{a}$
3. $\quad 1$
4. $\quad \dfrac{a}{b}$
5. $\quad \dfrac{b}{a}$

Choice B