the number of the students in class two is forty-five.这个句子中那个是谓语,怎么看得出来?

根据英语语法中的主谓一致原则,
the number of 是.的数量的意思,谓语动词必须接单数.
如：The number of the students in our school is 235.
a number of 是许多的意思,相当于a lot of.谓语动词用复数.
如：A number of people were injured in the terrible earthquake.
有不明白的请继续追问,