a. The story hasn't been completed, only the introduction.<br/><br/>b. The story hasn't been completed--only the introduction.<br/><br/>c. The story hasn't been completed; only the introduction has.<br/><br/>d. The story hasn't been completed. Only the introduction has.<br/><br/>e. The story hasn't been completed. Only the introduction has been completed.<br/><br/>Which do you accept, and which is your preferred?<br/><br/>I think I'm leaning towards c and d myself as my preferred, but I feel as though the most idiomatic would be a and b.