题目

It is thought that one billion people in the world,  ______is half the word’s workers, earn their living by farming. A. if            B. that           C. which             D. what   答案： B 句中“half the world’s workers”是对“one billion people in the world” 的补充说明，  “that is”是 “that is to say”的省略说法，是插入语，that 并不是引导非限制性定语从句的，如果在 “that is”后加个逗号则更清楚，应选B。

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