题目

已知四边形ABCD，对角线AC与BD交于点O，且AO=OC，DO=OB.求证：四边形ABCD是平行四边形. 答案：证明：由已知得,.∵,且A、D、B、C不在同一直线上.故四边形ABCD是平行四边形.—I like English very much. —What do you think        it ?A. of          B. in         C. on