Let $f ( x ) = x \sin \pi x , x > 0$. Then for all natural numbers $n , f ^ { \prime } ( x )$ vanishes at\\
(A) a unique point in the interval $\left( n , n + \frac { 1 } { 2 } \right)$\\
(B) a unique point in the interval $\left( n + \frac { 1 } { 2 } , n + 1 \right)$\\
(C) a unique point in the interval $( n , n + 1 )$\\
(D) two points in the interval $( n , n + 1 )$