题目内容
As is__________by scientists, global pollution has become one of the most serious problems humans are faced with.
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Competition for these jobs is very tough---we have five times as many ________ this year as we did last year.
The car burns too much gas, and ________, the price is almost twice as much as I intend to pay.
To understand a great international event, we need to consider the _________background to it.
It is hard to imagine how our forefathers could do_______so many conveniences that modern technology has brought about.
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