下列各式,属于二元一次方程的个数有( ) ①xy+2x-y=7;②4x+1=x-y;③;④x=y;⑤x2-y2=2 ⑥6x-2y ⑦x+y+z=1 ⑧y(y-1)=2y2-y2+x.
A.1 B.2 C.3 D.4