选D
这个主语从句有省略部分,完整的是：whether the experiment is worth doing (or not)正因为有括号里的成分,所以不能用if的这个“是否”.that和how都没有“是否”的意思.
It形式主语 ‘s系动词 uncertain表语 (whether) the experiment is worth doing.主语从句
It‘s uncertain () the experiment is worth doing. A.if B.that
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