Given an ellipse $C : \frac { x ^ { 2 } } { a ^ { 2 } } + \frac { y ^ { 2 } } { 4 } = 1$ with one focus at $( 2,0 )$, then the eccentricity of $C$ is
A. $\frac { 1 } { 3 }$\\
B. $\frac { 1 } { 2 }$\\
C. $\frac { \sqrt { 2 } } { 2 }$\\
D. $\frac { 2 \sqrt { 2 } } { 3 }$