16. It’s impolite to cut in line when you _______ a bus at the bus stop.
A. ask for B. look for C. care for D. wait for
17. —Can a plane fly _______the Atlantic Ocean?
—Yes, but it needs to go _______the clouds for hours.
A. across; through B. through; across
C. across; across D. through; through
18. —Mr Hu, can you tell us how to learn maths well?
—Sure. But remember nothing can be learned _______hard work.
A. by B. at C. without D. for
