Howling:K9 Mark III

Was a reason ever given as to why, when the doctor built K9 mark III, he gave it to sarah jane? Rather than keep it himself or any other companions because sarah was before k9's time and hadn't met him, onscreen at least. Also, when did he build it? Because he seemed to have the mark II ready for when mark I left with leela, but no indication was given for this model? I will also take production reasons like they wanted to write him out because I think that did happen. --90.199.70.162talk to me 23:36, May 10, 2014 (UTC)