Transcript text: Solve the given system by the substitution method. If there is no solution or an infinite number of solutions, so s Use set notation to express solution sets.
\[
\left\{\begin{array}{l}
x+3 y=6 \\
2 x+3 y=9
\end{array}\right.
\]
Select the correct choice below and, if necessary, fill in the answer box to complete your choice.
A. There is exactly one solution. The solution set is $\square$ 3.
(Type an ordered pair, using integers or fractions. Simplify your answer.)
B. There are infinitely many solutions. The solution set is $\{(x, y) \mid x+3 y=6\}$ or $\{(x, y) \mid 2 x+3 y=9\}$.
C. There is no solution. The solution set is the empty set, $\varnothing$.