将曲线y=sin3x变为y=2sinx的伸缩变换是(　　)

A：$\left\{\begin{array}{c}x=3x\text{'}\\ y=\frac{1}{2}y\text{'}\end{array}\right.$

B：$\left\{\begin{array}{c}x\text{'}=3x\\ y\text{'}=\frac{1}{2}y\end{array}\right.$

C：$\left\{\begin{array}{c}x=3x\text{'}\\ y=2y\text{'}\end{array}\right.$

D：$\left\{\begin{array}{c}x\text{'}=3x\\ y\text{'}=2y\end{array}\right.$