The impulse response h(t) of a linear time-invariant continuous time system is described by h(t) = exp(αt)u(t) + exp(βt)u(-t), where u(t) denotes the unit step function, and α and β are real constants. This system is stable if

A. α is positive and β is positive

B. α is negative and β is negative

C. α is positive and β is negative

D. α is negative and β is positive

Answer: Option D