一般来说，流行病学研究的起点是 A．描述流行病学研究 B．病例对照研究 C．队列研究

       Many places in the world need more fresh water. Every country is trying to find ways to turn salt water
into fresh. Why aren't there many factories like the Symi factory?
        In some places, the sun is not hot enough. Or it does not shine every day. In such places, other ways of
heating sea water can be used. These ways cost more money, but they work faster than the sun. By boiling sea
water with high heat, a lot of fresh water can be made quickly.
        But heating is not the only way to get fresh water from salt water. Other ways are tried. One way is
freezing. The fresh part of salt water freezes (冷至结冰) first. To get fresh water, the bits of ice are taken out. 
        Which way is the best? The one that gives the most water for the least money. It may be a different way
for each place.
        Symi's way seems very good for small, hot places. It doesn't make very much water at a time. But the
factory is easy to build and costs little. That is why people in many dry places talk about Symi!