2. 外语
  3. All over Europe, and in North America, there’re lakes and forests which are dead or (71) and the (72) is thought to be acid

All over Europe, and in North America, there’re lakes and forests which are dead or (71) and the (72) is thought to be acid

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问题    All over Europe, and in North America, there’re lakes and forests which are dead or  (71)  and the  (72)  is thought to be acid rain. What is acid rain? Mr. Justin Cook, a scientist, explains: "All fossil fuels, (矿物燃料)，that  (73)  oil, coal and gas, contain sulphur. When these are burnt, for example  (74)  motor vehicles, they form sulphuric acid (硫酸)This  (75)  UP into the air, and soon it falls back to  (76)  , into lakes and  (77)  trees in forests." This can cause many local problems.  78  a result, lakes become acidic, fish disappear and trees are killed. The pollution is carried great distances by the wind, so sulphuric acid  (79)  in Britain can travel as far as Scandinavia (斯堪的纳维亚). Czech is one of the countries with the most acid rains and large areas of  (80)  consist only  (81)  dead trees.
   Is there  (82)  that can be done about acid rain? Mr. cook says: "The rain  (83)  of acid rain is  (84)  power stations to install filters that  (85)  the acid getting out into the air. Here in Britain, the only  (86)  that has been taken is that the chimneys are built  (87)  so that the pollution is more distributed." If the petrol used in motor vehicles  (88)  lead--free, then it Would be possible to filter exhaust fuses (废气) which are a  (89)  cause of pollution. But these filter mechanisms (机械装置)can not work  (90)  the leads is removed.  
选项 A、if
B、when
C、until
D、unless
答案D
解析 句意：但是这些过滤机械装置不能起作用除非铅被去除掉。”四项中A是if表“如果”，B项when表示“当……时”，C项until表示“直到”，只有项unless(if...not)表示“除非，如果……不”。故选D为正确答案。
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