题目下列各式从左到右的变形中,为因式分解的是（ ）A.x(a−b)=ax−bxx\left(a-b\right)=ax-bxx(a−b)=ax−bxB.x2−1+y2=(x−1)(x+1)+y2x^{2}-1+y^{2}=\left(x-1\right)\left(x+1\right)+y^{2}x2−1+y2=(x−1)(x+1)+y2C.ax+bx+c=x(a+b)+cax+bx+c=x\left(a+b\right)+cax+bx+c=x(a+b)+cD.y2−1=(y+1)(y−1)y^{2}-1=\left(y+1\right)\left(y-1\right)y2−1=(y+1)(y−1)知识点：因式分解的应用章节：数与式 / 因式分解
