AHSME 1991 Problem 1

If for any three distinct numbers $a$, $b$, and $c$ we define $f(a,b,c)=\frac{c+a}{c-b}$, then $f(1,-2,-3)$ is $\textbf {(A) } -2 \textbf {(B) } -\frac{2}{5} \textbf {(C) } -\frac{1}{4} \textbf {(D) } \frac{2}{5} \textbf {(E) } 2$