10 students took part in a speech contest held in Grade Two, including 3 students in Class One, 2 students in Class Two and 5 students in other classes. If we draw lots, that's true.

There is an arrangement in which three students in one class are tied together and arranged with five students in other classes. Then two students from Class Two are inserted into seven gaps of eight arranged students, and there is an arrangement. So there is always an arrangement, and its probability is 0, so choose B.