one billion people in the world,中国人考中国人的奇怪选择题

1 whitch
B what
C that
it's thought that one billion people in the world, __ is half the world's workers, earn their living by farming
请详细回答`

It is thought that one billion people, that is half the world's workers, earn their living by farming.

据认为有十亿人,也就是全世界劳动人口的一半,是靠务农谋生的。

that is
那就是,即

二楼的对，他说的对。

which

你好！ 1.which 仅代表个人观点，不喜勿喷，谢谢。

which.非限定性定语从句。