they believe___D____ their duty to look after those children

They believe it their duty to look after those children whose parents died in the flood.
A.this 应该是(that)this is,因为要引导从句.
B.that 应该是that it is,也是由that引导宾语从句.
C.one 驴∧＿∧唇不对马嘴,非要用one的话→that it is one of
D.it后面的their duty to lo.是修饰解释it的,
比如:I insist it my right to gain it.

怎么只有it，应该是 they believe____that it is____their duty to look after those children whose parents died in the flood.翻译：他们相信那是他们的责任来照顾那些父母死在洪水中的孩子。
如果只有it的话就变成：他们相信它他们的责任来照顾那些父母死在洪水中的孩子。是错的！
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