18(63)Lord Krishna tells that He has divulged Guhyat Guhyataram now it is for Arjuna to activate this in his life or not he should ponder over it and then whatever his desire is he should act accordingly.here Bhagwan has given him freedom to act according tohis desire or not to act.But Arjuna had surrendered himself at the feet of Lord Krishna by sayingSisyteham Sadhi mam twam Prapannam 2(7)so how Arjuna could act of his own without getting clear mandate from Lord Krishna because once deciple surrenders(Sarnagati) he has to follow the mandate of Jagad Guru krishna and the mandate of sri Krishna has been continuously Nirashi Nirmamo bhutwa Yuddhaswa Bigat Jwarah3(30) so how Arjuna could act against this hence he did not respond and remained silent and the true mandate emerged in following Slokas to which Arjuna gave his willingness and acted upon.