If $f : \mathbb { R } \rightarrow \mathbb { R }$ is a twice differentiable function such that $f ^ { \prime \prime } ( x ) > 0$ for all $x \in \mathbb { R }$, and $f \left( \frac { 1 } { 2 } \right) = \frac { 1 } { 2 } , f ( 1 ) = 1$, then
[A] $f ^ { \prime } ( 1 ) \leq 0$
[B] $0 < f ^ { \prime } ( 1 ) \leq \frac { 1 } { 2 }$
[C] $\frac { 1 } { 2 } < f ^ { \prime } ( 1 ) \leq 1$
[D] $f ^ { \prime } ( 1 ) > 1$