— What on earth are the police busy with? — They have been looking into how the plane crash _______.

My teacher is very nice．I'll never forget the        he has done for me.

(2013·山东省胶东示范学校二模)—It's dangerous to take photos of lightning.—The challenge is not for ________， but I think it's worth  attempting.

The more one is ______ the English－speaking environment，the better he or she will learn the language.

I would rather          history than geography because I can learn a lot from many historical figures.