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
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(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