A function $f$ is continuous on the closed interval $[ 2,5 ]$ with $f ( 2 ) = 17$ and $f ( 5 ) = 17$. Which of the following additional conditions guarantees that there is a number $c$ in the open interval $( 2,5 )$ such that $f ^ { \prime } ( c ) = 0$ ?\\
(A) No additional conditions are necessary.\\
(B) $f$ has a relative extremum on the open interval $( 2,5 )$.\\
(C) $f$ is differentiable on the open interval $( 2,5 )$.\\
(D) $\int _ { 2 } ^ { 5 } f ( x ) d x$ exists.