A:4031
B:4033
C:4034
D:4032
下列从左到右的变形中是因式分解的有( )
①x2﹣y2﹣1=(x+y)(x﹣y)﹣1
②x4m+xm=xm(x3m+1)
③(x﹣y)2=x2﹣2xy+y2
④x2﹣9y2=(x+3y)(x﹣3y)