There’s some litter on the floor. Please ______.

My sister is only three years old. She can’t take care of _______. 

— What do you think of math? — I find               difficult to learn it.

--            is he?--He is PSY. He sang the song Gangnam Style (《江南Style》).

—Excuse me, but is that my passport?—Oh, sorry, I took ________ by mistake.