If the impulse response of a discrete-time system is $h[n]=-5^{n} u[-n-1]$, then the system function $\mathrm{H}(z)$ is equal to
- $\frac{-z}{z-5}$ and the system is stable
- $\frac{z}{z-5}$ and the system is stable
- $\frac{-z}{z-5}$ and the system is unstable
- $\frac{z}{z-5}$ and the system is unstable