It is necessary to explain your theory _____, otherwise the   audience won't make sense of what you are trying to get across.

—What were you doing when I rang you this morning?—I was occupied ______ the housework and kept my son occupied______ his homework.

— Do you think the MP3 player put its listener ________? — Yes, too much use of it could lead to hearing loss, experts say.

I have no money, so a new bicycle is ____.  

This book _________12 units, _________ the unit of Music.