第20题: [单项选择]转速的描述正确的是()。 A. 转速是指转动零件边缘某点单位时间内转过的线距离 B. r/min是指每秒钟转的圈数 C. n=50r/s是指转速为每秒转动50转 D. m/min是转速的单位 参考答案:C
第21题: [单项选择]He ______with Smith at least four times in the past three years. A. has been seen to meet B. was seen to meet C. had been seen meeting D. is seen meeting 参考答案:A 答案解析:本题测试被动语态的时态。从“in the past three years”可知,该句应用现在完成时,可排除B、C、D项。