Of you know what it was
a) every b)each c)whom d) one e)both

E i could see d as a option but if that was the it would have to say one of you know what it is

The unspoken rules  might be found in Pencey and New York are " Keep your mouth shut" and "don´t create problems".  These statements might be connected to feelings of isolation and disillusionment, to be as hide as possible in order to not to be seen.