In the rectangular prism $A B C D - A _ { 1 } B _ { 1 } C _ { 1 } D _ { 1 }$, $E$ is the midpoint of edge $C C _ { 1 }$. The tangent of the angle between skew lines $A E$ and $C D$ is\\
A. $\frac { \sqrt { 2 } } { 2 }$\\
B. $\frac { \sqrt { 3 } } { 2 }$\\
C. $\frac { \sqrt { 5 } } { 2 }$\\
D. $\frac { \sqrt { 7 } } { 2 }$