Not until all the fish died in the river ________ how serious the pollution was.
These instructions should be made more ___________ , so that the primary school students can understand it.
--- How long have you ___________? --- It’s about ten years since we ___________.
--- Who is the man ___________here? --- Mr. Black.
The stranger stopped and turned around ___________to see is someone was following him.
Where is it___________you picked up the wallet yesterday ?