某文具店一本练习本和一支水笔的单价合计为3元，小妮在该店买了20本练习本和10支水笔，共花了36元．如果设练习本每本为 $x$元，水笔每支为 $y$元，那么根据题意，下列方程组中，正确的是 $($　　 $)$

A． $\left\{\begin{array}{c}x-y=3\\ 20x+10y=36\end{array}\right.$B． $\left\{\begin{array}{c}x+y=3\\ 20x+10y=36\end{array}\right.$

C． $\left\{\begin{array}{c}y-x=3\\ 20x+10y=36\end{array}\right.$D． $\left\{\begin{array}{c}x+y=3\\ 10x+20y=36\end{array}\right.$