The impulse response $h(t)$ of a linear time-invariant continuous time system is described by $h(t)=\exp (\alpha t) u(t)+\exp (\beta t) u(-t)$, where $u(t)$ denotes the unit step function, and $\alpha$ and $\beta$ are real constants. This system is stable if
- $\alpha$ is positive and $\beta$ is positive
- $\alpha$ is negative and $\beta$ is negative
- $\alpha$ is positive and $\beta$ is negative
- $\alpha$ is negative and $\beta$ is positive