选择为什么不选A????

因为the teacher 是主语，his students是主语的跟随语，谓语要随主语的时态和形式来填写

主语是老师 单数

the teacher 为主语 是单数
with his students是附带
此句可变为
the teacher is swimming over there with his students.

主语是the teacher,with his students是插入语，所以谓语用is,选B

因为with his students是伴随着主语的，是the teacher的插入语，做语法时不要看with his students，类似还有as.......as
等一些词组