Humanity uses a little less than half the water available worldwide. Yet occurrences of shortages and droughts are causing fami

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问题    Humanity uses a little less than half the water available worldwide.
Yet occurrences of shortages and droughts are causing famine in some
areas, but industrial and agricultural by-products are polluting water supplies. 【S1】______
Since the world’s population is expecting to double in the next 50 years, 【S2】______
many experts think we are on the edge of a widespread water crisis.
   But that doesn’t have to be the outcome. Water shortages do not
have to trouble the world—if we started valuing water more than we 【S3】______
have in the past. Just like we began to appreciate petroleum more after 【S4】______
the 1970s oil crisis, today we must start looking at water from a fresh
economical perspective. We can no longer afford to consider water a 【S5】______
virtual free resource of which we can use as much as we like. 【S6】______
   Instead, for all uses except the domestic demand of the poor,
governments should price water to reflect their actual value. This means 【S7】______
charging a fee for the water itself as well as for the demand costs. 【S8】______
   Governments should also protect this resource by providing
water with more economically and environmentally sound ways. 【S9】______
Often the cheapest way to provide irrigation water in the dry tropics
is through small-scaled projects, such as gathering rainfall in depressions 【S10】______
and pumping it to nearby cropland.
【S3】

选项

答案started—start

解析 时态错误。主句使用了一般现在时do not have to trouble,因此if从句里的一般过去时要改成一般现在时。
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