Trainer Aptitude

Which of the following is least like the others?

Consider a language which uses the following set of characters: Small set: { a b c } Large set: { A B C } Punctuation set: { x y } This language must follow the following rules: 1. A punctuation character must end all series. 2. A series can have up to but no more than 4 characters, including punctuation characters. Does the following series follow all the rules of the language defined above? axBy