.___ is our duty for us young adults to work hard at our lessons.A.That B.It C.This D.He
问题描述：
.___ is our duty for us young adults to work hard at our lessons.A.That B.It C.This D.He
答
B it
It 在此句中作形式主语,真正主语是后面的动词不定式复合结构for us young adults to work hard at our lessons