Yes, Karan knows that's why he is quiet and he is saving that killer...he knows it's not nandini, becz before the party and in the party they had argument and after argument karan left by saying that he will kill Gujral but he was drunk and we all know that it's not karan....and Mihir said by him self that he is not good father that he is letting his son dieing and he is helpless, he couldn't do anything...he is not that good as Karan, karan knows the killer that' why he is quiet and saying nothing, he reacted different becz Karan knows that Tulsi and Tanya are searching for killer so he might thought that it could be Tulsi's plan that by saying nandini's name karan would react different as he did but he would say something like, "no it's not nandini it's _____ person who killed gujral"...but actually he didn't say anything...and we all know that is' not karan neither nandini...but no body has suspect mihir's reaction yet as they suspect gautam, mohini, karan, nandini...if mihir will speak up that he is the killer than everything will be finished in virani house, he is the main in virani house..he is really upset this time becz if he is the killer than he is not supporting the truth..he is hiding. wat do u think?