下列方程组中，二元一次方程组是（   ）

A：$\{\begin{array}{l}2xy\text{=5}\\ 2x-y\text{=6}\end{array}$

B：$\{\begin{array}{l}x+y\text{=2}\\ x-y\text{=0}\end{array}$

C：$\{\begin{array}{l}2x-y\text{=1}\\ y\text{=3z+1}\end{array}$

D：$\{\begin{array}{l}{x}^2\text{=1}\\ y-1\text{=x}\end{array}$