We also had different students in some classes so was a struggle to remember all the fac

问题补充：

We also had different students in some classes, so was a struggle to remember all the faces.

A. it

B. this

C. that

D. one

答案：

(答案→)A. it

B. this

C. that

D. one

解析：考查it做形式主语：句意：我们一些班级有不同的学生，所以，记住他们的名字很难。这里使用：It is/was+adj/n+to do…，这里it是形式主语，不定式是真正的主语，选A