The people on the Island of Knights, Knaves, and Normals, are either knights, knaves, or normals.
Knights are honest and virtuous, and whatever they say is true.
There are three people, a knight, a knave, and a normal. Solution A cannot be a knight since then he would be lying and C cannot be a knave since then he would speak the truth which knaves just don't do. Suppose A was a normal. That would make B a knight and so C would have to be a knave. But that can't be.