Solve the System by the Addition Method $$\left\{ \begin{array} { l } x + y = 7 \\x - y = 1\end{array} \right.$$

Multiple Choice

Solve the system by the addition method. If there is no solution or an infinite number of solutions, so state. Use set
notation to express the solution set.
- $$\left\{ \begin{array} { l } x + y = 7 \\x - y = 1\end{array} \right.$$

A) $\{ ( 3,4 ) \}$
B) $\{ ( - 4,4 ) \}$
C) no solution; $\varnothing$
D) $\{ ( 4,3 ) \}$