[单项选择](The age of) a geological sample can (be estimated) from the ratio of radioactive to nonradioactive carbon (present) in the object (is examined). A. The age of B. be estimated C. present D. is examined
参考答案:D 答案解析:英语句子一般只有一个中心动词,除非句子中出现and或者“;”。此题题干中已经出现了中心动词can be estimated,其他的动词只能换成非谓语动词的形式,即现在分词、过去分词或动词不定式。应将i