It is well known to us all that a lot of rivers _______by factories.

Helen ______ her keys in the office so she had to wait until her husband ______ home. 

(2014·江苏盐城二模)Lewin was interested in taking research beyond books and looking at how it could________real social change.

The performance of the host ,______to please the audience and draw their attention ,was greeted with a cold silence ,however.

A new kind of drug ___ ___ to cure H7N9 bird flu by the scientists and they are hopeful that they will succeed in a couple of weeks.