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。
