The impulse response h(t) of a continuous time, linear time invariant system is described by h(t) = eat u(t) + ebt u(-t) where, u(t) denots the unit step function and a and b are real constants. This system is stable if

• A. both a and b are positive
• B. a is positive and b is negative
• C. a is negative and b is positive
• D. both a and b are negative