题目

As far as I know, the robot that can sweep the floor ________ in the 1980s.A. is invented B. invented C. was invented D. has invented 答案：【答案】C【解析】句意：据我所知，能扫地的机器人是在20世纪80年代发明的。考查一般过去时的被动语态。A. is invented一般现在时的被动语态；B. invented过去式或过去分词；C. was invented一般过去时的被动语态；D. has invented现在完成时。根据题干可知，句子的主语the robot是谓语动词invent的承受者，构成被等腰三角形的周长是25cm，一腰上的中线将周长分为3：2两部分，则此三角形的底边长为 cm或 cm．