问题补充:
It is thought that one billion people I the world, is half the word’s workers, earn their living by farming.
A. if
B. that
C. which
D. what
答案:
A. if
(答案→)B. that
C. which
D. what
解析: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。