第21题: [单项选择](Leading) scientists are often the (kinds of person) who (have enjoyed) intellectual challenges (all their lives). A. Leading B. kinds of person C. have enjoyed D. all their lives 参考答案:B 答案解析:名词单复数错误,应改为kind of persons。主语scientists是复数,其表语也应是相应的 persons。由于scientists只是一类人,kind用单数就行了。