Solve the Problem $$\left\{ \begin{array} { l } x = - y \\x + y = 6\end{array} \right.$$

Multiple Choice

Solve the problem.
- $$\left\{ \begin{array} { l } x = - y \\x + y = 6\end{array} \right.$$

A) $\{ ( 1,5 ) \}$
B) no solution; $\varnothing$
C) infinitely many solutions; $\{ ( x , y ) \mid x + y = 6 \}$ or $\{ ( x , y ) \mid x = - y \}$
D) $\{ ( 1,1 ) \}$