The stduents in Class One are quieter than -- in Class Three

The stduents in Class One are quieter than __ in Class Three.
A.ones B.those C.them D.that
选B
因为
The stduents in Class One are quieter than
those 指代三班学生
those是代词
再问： 为什么不用that？that 不是代词吗？还是因为前面是复数？
再答： 前面是复数