A sandstorm, also known as a dust storm, is exactly what it sounds like. A very strong windstorm especially in the desert carries clouds of sand or dust that greatly reduces visibility. This wind is usually caused by convection currents (对流气流), which are created by intense heating of the ground, and is usually strong enough to move entire sand dunes. Air is unstable when heated, and this instability in the air will cause higher winds to mix with winds in the lower atmosphere, producing strong surface winds.
Sandstorms can disturb people’s travel, and can sometimes destroy whole roads, and dry flat areas. Similar dust storms can be found on the planet Mars, and are thought to be seasonal. Today, the destruction of forests and too much farming of farmland can lead to sandstorms. Too much use of water resources can also cause sandstorms.
In the United States of America, sandstorms are rare because of the lack of large deserts and more importantly the development of proper agricultural techniques. The last recorded destructive sandstorm in American history was the Dust Bowl, which occurred on July 16, 1971 and was widely recorded by meteorologists (气象学家).
To protect themselves from sandstorms, some people wear protective suits. Special equipment can be fixed in some cars to prevent sand from getting into the engine. Sandstorms can cause coughs, and the sand and dust has also been known to carry “infectious diseases”. Sand particles, unlike dust ones, will block air passages, and cause a person who breathes them in to choke. Dust particles may cause an allergic (过敏的) reaction.
Which of the following plays the first part in forming sandstorms?

A．The instability of the sand.

B．The instability of the wind.

C．The intense heating of the ground.

D．The speed of the wind.

From the second paragraph, we can learn that at present _______.

A．sandstorms are more often seen on Mars than on Earth

B．sandstorms have led to the lack of clean water resources

C．farming brings more harm than benefits to humans

D．human activities are a major cause of sandstorms

The author will most probably agree that ________.

A．there will soon be no sandstorms in the USA

B．humans haven’t thought of any way to deal with sandstorms

C．death caused by sandstorms is increasing all the time

D．humans should make more efforts to stop soil turning into desert