---People often don’t appreciate what they have, do they? ---No, _______ they lose it.
A. if B. after
C. until D. when
It’ll be another 50 years _________ China is expected to reach the level of a medium
developed country.
A. since B. before
C. that D. when
General Secretary, Xi Jinping, stressed that cross－strait cooperation would be strengthened________ the two sides could agree to oppose Taiwan independence．
A． unless B． even if
C． as long as D． until
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）.
