We can never expect bluer sky unless we create less polluted world.
The development of industry has been ______ gradual process throughout ______ human existence, from stone tools to modern technology.
We should get prepared for dealing with ________problems_________.
---You’ve read the Frog by Moyan , haven’t you?---Yes, I_______ second time and I got a better understanding.
We’ll never forget this terrible accident and the damage_______ has caused to the area.
--- You ought to have given them some advice.--- , but who cared what I asked?