已知甲、乙两数的和是7，甲数是乙数的2倍．设甲数为x，乙数为y，根据题意，列方程组正确的是（  ）

A：$\{\begin{array}{l}x+y=7\\ x=2y\end{array}$

B：$\{\begin{array}{l}x+y=7\\ y=2x\end{array}$

C：$\{\begin{array}{l}x+2y=7\\ x=2y\end{array}$

D：$\{\begin{array}{l}2x+y=7\\ y=2x\end{array}$