Suppose \( f(x)=e^{a x}+e^{b x} \), where \( a \neq b \), and that \( f^{\prime \prime}(x) \) \(...

Suppose \( f(x)=e^{a x}+e^{b x} \), where \( a \neq b \), and that \( f^{\prime \prime}(x) \) \( -2 f^{\prime}(x)-15 f(x)=0 \) for all \( x \). Then the product \( a b \) is
(A) 25
(B) 9
(C) \( -15 \)
(D) \( -9 \)