Let $A=\left(\begin{array}{ccc}-1 & 3 & 4 \\ 0 & 5 & 8 \\ 0 & 0 & 1\end{array}\right)$ and $B=\left(\begin{array}{ccc}5 & 0 & 0 \\ 0 & -4 & 0 \\ 0 & 0 & -1\end{array}\right)$. The characteristic values of $A B$ are
(a) $-5,6,-1$
(b) $5,-4,-1$
(c) $-5,-20,-1$
(d) $-4,1,0$