甲乙两地相距126 km，一辆小汽车从甲地开往乙地，一辆货车从乙地开往甲地，两车同时出发相向而行，经过45分钟两车相遇，相遇时小汽车比货车多行6km，设小汽车和货车的速度分别为xkm／h、ykm／h，则下列方程组正确的是（    ）

A：$\left\{\begin{array}{c}45\left(x+y\right)=126\\ 45\left(x-y\right)=6\end{array}\right.$

B：$\left\{\begin{array}{c}\frac{3}{4}\left(x+y\right)=126\\ x-y=6\end{array}\right.$

C：$\left\{\begin{array}{c}\frac{3}{4}\left(x+y\right)=126\\ 45\left(x-y\right)=6\end{array}\right.$

D：$\left\{\begin{array}{c}\frac{3}{4}\left(x+y\right)=126\\ \frac{3}{4}\left(x-y\right)=6\end{array}\right.$