Notice that with 8 chairs, there are 4 pairs of chairs opposite each other. The two students can occupy each of those 4 pairs in 2 ways. For instance, one student can be in chair 1 and the other in chair 5, but then they can switch, with the first student in chair 5 and the other student in chair 1. This way, there can be 4*2 = 8 different arrangements, so B is the answer.