28. The water will be further polluted unless some measures ______.
That scientist ________ work is successful has been made a model worker.
Many athletes are clear that the Olympic Games are the only time ________ they will be watched by thousands of millions.
The old man often looked back on the past days ________ he worked hard to make a living.
Recently I have bought an iPad 3 on the website of taobao. com, ________ the price is very reasonable.
Have you ever seen any trees ________ leaves were black because of the polluted air?