It is 10 o ' clock ______ my watch.
If the answer is BY, please explain me why.
'By' is a preposition that expresses proximity(by the riverside), means(come by train), action with a purpose( passed by working hard), a person's action in passive voice( done by him). 'It is ten o'clock by my watch' expresses that according to my watch, the time is ten o'clock.