One easy way to extend the thinking time for students to answer questions is to ask:'Are there any other bids?' or maybe "Are there other possibilities?"  By doing this, even when the first response may have been correct students quickly learn that they are free to agree or disagree with the first responder. After doing this several times I don't have to ask the second questin any more.  Just by delaying my response the class begins to contribute more opinions to the discussion.  We can then discuss which ways were the better ways to look at the problem and can clear up any misconceptions without singling anyone out.