答案：A.
翻译：这所学校学生的数量比他们学校学生数量要更大/多.
1.that为指示代词,代指前面的the number of the students,而theirs = their school.
即原句可以改写为：The number of the students in this school is larger than that in theirs.
= The number of the students in this school is larger than the number of the students in their school.
2.而B项的of mine没法理解,mine倒是可以等于my school,但是介词of有误,和前文不对应.
前句of the students,但是mine不能换成the students,只能换成my school.13.The number of teachers in our school is larger thanof theirs.A .it B. thisC. thatD.those不止一篇资料出现过这样的题干。该如何解释呢这个题目答案还是C that.比较必须是同类事物比较，所以只能用that代替the number of the students.此题出题有失严谨，最好是that in theirs.建议楼主不要太相信一些参考资料。我也是觉得不妥，想问问清楚。谢谢我认为还应该是in theirs，不是of theirs.我刚才搜索过，说成that in theirs的更多。