Not until all the fish died in the river, _______ how serious the pollution is.[ ]

Finding the ______look on the student’ s face, the professor felt _____about his explanation.

He is very grateful to the girl____ he often turns for help when he meets with difficulties.

Not only ______ the book, but he remembered what he had read.

Whatever you have found，you should give it back to whomever it ________