Guys, I think Arnav slaps Shyam not because he knows the truth..well he knows but as we all know he didn't believe it. So, not when Shyam blames Khushi for trapping him, this time Arnav very well remembers how Shyam told him that he loves Khushi and how Khushi too love him unless NK comes and clears the MU before they enter the RM Or it may be because his very deep inside the heart he believed Khushi and as today he actually asked her whether Khushi has proofe to prove before all so he immediately trusts her...
65