题目内容：

    If you live in a desert, maintaining a supply of fresh water is a challenge. One answer is desalination, but that needs a source from which to get the salt away—which in turn requires that your desert be near the sea. The other is related to moisture. Even in inland deserts, though, moisture is often present in the air as water vapour (水蒸气).

The problem is how to get this vapour effectively and cheaply from the desert air. And that is what two groups of researchers have managed to do.

The ease with which water can be won from air depends on that air’s relative humidity. This is a measure of its current vapour content as a percentage of its maximum possible vapour content at its current temperature. A relative humidity of 100% means the air in question is holding as much water vapour as it possibly can. A good way to get air to give up some of its moisture is therefore to cool it to the point where its relative humidity is more than 100%. Sometimes this happens naturally at night, causing mist and dew to form. These can be collected in special traps in areas where liquid water is otherwise rarer. But, if night cooling does not bring air all the way up to 100% relative humidity, building water traps out of special materials might give nature a helping hand.

Adsorption (吸附) is a process which pulls water molecules from air that has less than 100% relative humidity by attaching them to the surface of a solid material. The molecules are held there by electrostatic (静电的) connections called Van der Waals forces that link them with the molecules of the related surface. To collect a lot of water this way therefore requires a material that has two features. One is a large surface area. The other is an appropriate Van der Waals response. Experimental traps that employ this principle have been made using substances called metal-organic frameworks. These are porous (多孔的) molecular networks through which air can circulate. Their porosity gives them a huge surface area. And by picking the right ingredients, such as zirconium, they can be given the necessary Van der Waals features. Zirconium is, however, costly. Moreover, once absorbed, the water must then be released. This means warming the absorptive material—the warmth being provided by the sun, once it has risen. Here, metal-organic frameworks present a problem. They tend to reflect sunlight rather than absorb it, and so don’t heat up well. To overcome this, engineers build a solid device made of copper into the system. This works, but adding such devices makes an already costly technology even dearer.

1.What is the meaning of the underlined word in Paragraph 1?

A.The irrigation of sea water. B.The import of sea water.

C.The removal of salt in sea water. D.The purification of sea water.

2.Which factor can mainly affect people to get water from the air easily?

A.Air quality. B.Relative humidity.

C.Current vapour content. D.Current temperature.

3.What can we learn from the article?

A.Water drops are the major existence of moisture in the air.

B.Getting vapour in the air naturally usually happens when the relative humidity is below 100%.

C.The more holes a certain material has, the huger surface area it has.

D.Zirconium is chosen because it’s cheap.

4.What’s the main idea of the passage?

A.Maintaining-fresh water in the desert.

B.Introducing a way to get water in the air of the desert.

C.Promoting an experimental trap to get water in the air.

D.Offering a way to increase adsorption in the air.