__we are doing has never been done beforeA that B what
问题描述：
__we are doing has never been done before
A that B what
答
选B，很明显，是what 引导的主语从句，what we are doing这个句子作为整个句子的主语部分，表示我们正在做的事情是以前从来没有做过的。
答
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
答
B what
that没有意思，不能充当成分，所以选B
答
选 B
What 引导主语从句，指代我们正在做的事。
我们正在做的事以前从未做过。
答
选 B
What 引导主语从句，指代我们正在做的事
答
What 我们正做的事情以前从来没做过。主语从句用what
答
___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...
答
B what what 引导主语从句。