下列方程组中，是二元一次方程组的是（　　）

A：$\left\{\begin{array}{c}x+y=3\\ z+x=5\end{array}\right.$

B：$\left\{\begin{array}{c}x+y=5\\ y^2=4\end{array}\right.$

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

D：$\left\{\begin{array}{c}x=y+11\\ x^2-2x=y+x^2\end{array}\right.$​