___is still unknown when we can finish this project.a,it.b,that.c,which.d,this.
问题描述：
___is still unknown when we can finish this project.
a,it.b,that.c,which.d,this.
答
A
答
A it 用来做形式主语 后面的从句when we can finish this project是真正的主语，但是主语太长 未来避免头重脚轻 用it来作形式主语
答
a it 形式主语 when we can finish this project is still unkown
it指代when we can finish this project