用is和are都对

有一读者问： 如何做这道选择题：

The teacher together with his students_______the Great Wall.

A. is going visit　B. are going visit

C. is to go to visit　D. are to go to visit

根据语法规则，应选C。但D也不错，这是由于人们往往把together with混同为and的缘故。