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Two non-zero real numbers, $a$ and $b$, satisfy $a b=a-b$. Which of the following is a possible value of $\frac{a}{b}+\frac{b}{a}-a b$? (2000 AMC 12 Problems, Question \#11) 
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$$\frac{a}{b}+\frac{b}{a}-a b=\frac{a^{2}+b^{2}}{a b}-(a-b)=\frac{a^{2}+b^{2}}{a-b}-\frac{(a-b)^{2}}{(a-b)}=\frac{2 a}{a-b}=\frac{2(a-b)}{a-b}=2 \Rightarrow \text{E}.$$ 