已知数列(xn)满足Xn+1=Xn-Xn-1(n大于等于2),x1=a,x2=b,记Sn=x1+x2+…+xn,则下列结论正确的是(）A.X100=-a,S100=2b-aB.X100=-b,S100=2b-aC.X100=-b,S100=b-aD.X100=-a,S100=b-a请简要写出解答过程.谢谢!!!