(2007?嘉定区二模)如图:在梯形ABCD中,AD∥BC,BC=2AD,对角线AC与BD相交于点O,把△ABO,△BCO,△COD,△DOA的面积分别记作S1,S2,S3,S4,则下列结论中,正确的是( )A.S2=4S1B.S2=3S1C.S1=S3D.S1+S3=S2+S4