Mankind must first of all eat, drink, have shelter and clothing ________ it can pursue politics, science, art and religion.
A. until B. unless
C. before D. since
Will it be ages ______ he goes back to school? I'm so worried about him.
A. before B. after
C. when D. as
阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。
The way we cook is important. In many countries, the two choices are 1. (nature) gas or electric-powered stoves.
The World Health Organization warns that millions of people 2. (suffer) from indoor air pollution at present time, which results from the use of 3. (danger) fuels and cook-stoves in the home.
WHO officials say nearly three billion people are 4. (able) to use clean fuels and technologies for cooking ，heating and lighting. As a result, more than seven million people die from exposure to indoor or outdoor air pollution each year. Most of the deaths are in 5. （develop） countries, such as India, China and Latin American countries.
WHO officials say opening a window or door 6. （let）out the harmful air will not correct the situation 7. will only pollute the outdoors.
Nigel Bruce, 8. is a professor of Public Health at the University of Liverpool, says researchers are developing good cook-stoves and other equipment to burn fuels 9. a more efficient way.
But, this is just a start. It is urging developing countries to use 10. (clean) fuels and increase access to cleaner and more modern cooking and heating appliances（用具）.
________ the job takes a significant amount of time, most students agree that the experience is worth it.
A. If only B. After
C．Although D．In case
______ scientists have learned a lot about the universe, there is much we still don't know.
A. Once B. Since
C. Though D. Unless
_____ he once felt like giving up, he now has the determination to push further and keep on going.
A. Where B. As
C. In case D. Now that
