下列不等式组是一元一次不等式组的是（ ）
A. $\left\{\begin{array}{l}x+y=1 \\ x-y>1\end{array}\right.$
B. $\left\{\begin{array}{l}x^2+x>2 \\ x+1>0\end{array}\right.$
C. $\left\{\begin{array}{l}2 x+x>x \\ x+2>3 y\end{array}\right.$
D. $\left\{\begin{array}{l}x+1 < 2 \\ 2 x+3>x-1 \\ x>2\end{array}\right.$