Question

__we are doing has never been done before

Answer 1

___we
are
doing
has
never
been
done
before.
B，What
是正确答案
[What]
we
are
doing
has
never
been
done
before.
✓这样的读法是正确的
[That]
we
are
doing
has
never
been
done
before.
✗如果用'that’那么就不对了，如果在这个句子里加'subject‘（That
subject
we
are
doing
has
never
been
done
before），那么就对了。但是这个句子里无类似'subject’的词，因此我们不选A
为了完整了解，特地翻译成中文。以下是正确翻译：
我们正在做的事是以前从未做过的。

Answer 2

选B
这里what指代的内容既是从句we
are
doing的宾语
也是主句的主语
that没有这一功能
我们正在做的事情之前从来没有被完成过

Answer 3

what
we
are
doing
has
never
been
done
before.
其中what
we
are
doing是一个主语从句，what又是从句中的宾语，不可用that。