Solve the Nonlinear System of Equations for Real Solutions $\left\{ \begin{array} { l } x ^ { 2 } - y ^ { 2 } = 15 \\x ^ { 2 } + y ^ { 2 } = 21\end{array} \right.$

Multiple Choice

Solve the nonlinear system of equations for real solutions.
- $\left\{ \begin{array} { l } x ^ { 2 } - y ^ { 2 } = 15 \\x ^ { 2 } + y ^ { 2 } = 21\end{array} \right.$

A) $\varnothing$
B) $( - 3 \sqrt { 2 } , - \sqrt { 3 } )$
C) $( 3 \sqrt { 2 } , \sqrt { 3 } ) , ( - 3 \sqrt { 2 } , \sqrt { 3 } )$
D) $( 3 \sqrt { 2 } , \sqrt { 3 } ) , ( - 3 \sqrt { 2 } , \sqrt { 3 } ) , ( 3 \sqrt { 2 } , - \sqrt { 3 } ) , ( - 3 \sqrt { 2 } , - \sqrt { 3 } )$