【正确答案】 C

【答案解析】[理由] 本句测试K代替名词从句作主语的用法。
名词从句共有3种
1) that从句(任何陈述句之前冠以that, 即成that从句)
[例] He doesn't believe my words.(句子)
→that he doesn't believe my words
(名词从句)
(他不相信我说的话)
2) whether从句(任何可用yes/no回答的问句前冠以whether，并将倒装的问句还原)
[例] Is he happy?(问句)
→whether he is happy(名词从句)
(他是否快乐)
Can he do it?(问句)
→whether he can do it(名词从句)
(他是否能做这件事)
Did he come?(问句)
→whether he came(名词从句)
(他是否来了)
3) 疑问词从句(由疑问词when、what、how、where、why等引导的问句变化而成)
[例] What is he doing?(问句)
→what he is doing(名词从句)
(他正在做什么)
Where can he find it?(问句)
→where he can find it(名词从句)
(他在哪里找到它的)
What did he write?(问句)
→what he wrote(名词从句)
(他写了什么)
名词从句在句中可当主语或宾语。
[例] That he is a good student(主语) is true.
(他是好学生这件事是真的。)
Whether he will come or not(主语) remains to be seen.
(他会不会来还是个未知数。)
What he just said(主语) was hard to understand．
(他刚刚说的话很难懂。)
I believe that he is a good student.(宾语)
(我相信他是个好学生。)
I don't know whether he will come or not.(宾语)
(我不知道他是否会来。)
I don't understand what he just said(宾语).
(我不了解他刚说的话。)
名词从句作主语时，可用形式主语it代替，故上列名词从句均可改写如下：
It is true that he is a good student.
It remains to be seen whether he will come or not.
It was hard to understand what he just said.
原句空格后有“whether…or not”引导的名词从句，故知空格内应置入代词it作形式主语，以代替该名词从句。因此(C)应为正选。