16. If $f ( x ) = \sin \left( e ^ { - x } \right)$, then $f ^ { \prime } ( x ) =$\\
(A) $- \cos \left( e ^ { - x } \right)$\\
(B) $\cos \left( e ^ { - x } \right) + e ^ { - x }$\\
(C) $\cos \left( e ^ { - x } \right) - e ^ { - x }$\\
(D) $e ^ { - x } \cos \left( e ^ { - x } \right)$\\
(E) $- e ^ { - x } \cos \left( e ^ { - x } \right)$\\
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