I think it's important not to overwhelm students with facts they aren't interested in and which are not necessary for the lesson to continue. In the situation you have described, it might be better to keep the other answers to yourself, unless students ask about them. Perhaps just mentioning that other answers are possible will suffice. Either it will arouse the students' interest, and they will become willing to learn, or it won't, in which case there will be no harm in going on with the lesson. If, however, a student comes up with a correct aswer that the coursebook doesn't mention, they should certainly be commended and the answer should be commented on. This always gives a good chance of making a student remember something in my opinion.