已知数列{xn}满足xn+1=xn-xn-1(n≥2),x1=a,x2=b,Sn=x1+x2+…+xn,则下面正确的是( )A.x100=-a,S100=2b-aB.x100=-b,S100=2b-aC.x100=-b,S100=b-aD.x100=-a,S100=b-a