题目一个两位数x,还有一个两位数y,若把两位数x放在y前面,组成一个四位数,则这个四位数为( )A.10x+y B.xy C.100x+y D.1000x+yA.10x+yB.xyC.100x+yD.1000x+y知识点:列代数式章节:整式的加减