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

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问题     Humanity uses a little leas 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.【M1】______
    Since the world’s population is expecting to double in the next 50 years,       【M2】______
    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             【M3】______
    have in the past. Just like we began to appreciate petroleum more after         【M4】______
    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             【M5】______
    virtual free resource of which we can use as much as we like.                   【M6】______
    Instead, for all uses except the domestic demand of the poor,
    governments should price water to reflect their actual value. This means        【M7】______
    charging a fee for the water itself as well as for the demand costs.            【M8】______
    Governments  should also protect this resource by providing
    water with more  economically and environmentally sound ways.                   【M9】______
    Often the cheapest way to provide irrigation water in the dry tropics
    is through small-scaled projects, such as gathering rainfall in depressions     【M10】______
    and pumping it to nearby cropland.
【M3】

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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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