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) 
选项为:A:$-2$  B:$\frac{-1}{2}$  C:$\frac{1}{3}$  D:$\frac{1}{2}$  E:$$2$$  请回答其中一个选项.