The impulse response h(t) of a linear time-invariant continuous time system is described by h(t) = exp `(alphat)`u(t) + exp `(betat)`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 negative
`alpha` is negative and `beta` is negative
`alpha` ` ` is positive and `beta` is negative
`alpha` is negative and `beta` is positive