Verbal | Re: At the end of the week long conference, students from all local school

At the end of the week long conference, students from all local schools performed a cultural program.

Select the correct choice if the performance took place before the conference ended

A. At the end of the week long conference, students from all local schools performed.

B. At the end of the week long conference, students from all local schools had performed.

C.By the time conference ended, students from all local schools had performed.

Correct Answer is C but I dont get why B is wrong?

Guys