4.已知关于x,y的方程组$\left\{{\begin{array}{l}{ax+by=10}\\{mx-ny=8}\end{array}}\right.$的解是$\left\{{\begin{array}{l}{x=1}\\{y=2}\end{array}}\right.$,则关于x,y的方程组$\left\{{\begin{array}{l}{\frac{1}{2}a(x+y)+\frac{1}{3}b(x-y)=10}\\{\frac{1}{2}m(x+y)-\frac{1}{3}n(x-y)=8}\end{array}}\right.$的解为( )
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| A. | $\left\{{\begin{array}{l}{x=1}\\{y=2}\end{array}}\right.$ | B. | $\left\{{\begin{array}{l}{x=2}\\{y=1}\end{array}}\right.$ | C. | $\left\{{\begin{array}{l}{x=4}\\{y=-2}\end{array}}\right.$ | D. | $\left\{{\begin{array}{l}{x=3}\\{y=2}\end{array}}\right.$ |