题目内容
It's already tens of years ________ the world's population reached five billion.
A. before B. after
C. since D. until
C 【解析】句意：自从世界人口达到五十亿以来，已经有几十年了。考查连词辨析题。四个选项都可用于表示时间。A. before在……之前；B. after在……之后；C. since自从，后接时间段或从句，表示一段时间；D. until直到……才，多与not连用。本句it是形式主语，since引导的从句是真正主语，根据句意结构和语境，可知ABD三项意思都与句意不合，故选C。
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The population of Earth is still going up. There will be less and less good land for growing crops in the future. People worry that there will not be enough food for everyone in the middle of this century.
Recently scientists have discovered one strange plant
named fedtschenkoi（玉吊钟）may help. It can absorb（吸收）carbon dioxide(CO₂)
at night. It only needs 10 times less water than food plants like beans. This means it
can grow in the desert(沙漠).
In which ways could this plant help us?
Since deserts are increasing in size because of the changes in Earth’s climate, growing plants there will be more and more important. Although it is dangerous to
eat this plant, scientists can still study something about it: how it absorbs CO₂ at
night while many other plants only do this during the day, and why it is suited to the
desert.
There is another direction to take the research in. Producing a desert plant to eat
will be difficult, but there is still possibility to use the genes(基因)from this plant and put them into food plants.
The other way is to grow this type of plant for fuel(燃料). At the moment we
grow plants for fuel in good land. If we do it in deserts instead, more good land can be used to grow plants for food.
Desert plants have special qualities, which could be an answer to our future food
or energy needs. Scientists are working hard on it.
The Desert Plant Could Help | |
The problems of the future | ◆The population on Earth is becoming 1. than before. ◆There is less and less 2. land for growing crops. ◆There will not be enough food in the future, which makes people 3. |
The discovery of the scientists | ◆It is an 4. plant named fedtschenkoi. ◆Food plants like beans need 10 times 5. water than it does in deserts. |
The ways it could 6. us | ◆The scientists are studying how it absorbs CO₂ and why it is 7. for living in desert. ◆It is 8. to use genes from one plant and then put it inside another. ◆It will be better if we can grow plants for fuel in deserts 9. than in good land. |
Conclusion | ◆Because desert plants have special qualities, it may 10. our food and energy problems in the future. |