What’s going on?

　　The Earth is covered in a layer of air that is nearly 80 miles thick and at sea 1evel exerts or“pushes”hard almost 15

　　pounds of pressure per square inch.That means that a full sheet of newspaper laid out flat has nearly 9，300 pounds of air above it.

　　When you break the ruler above，you are able to break it because of the“heavy”air pushing down on the paper while you quickly strike the ruler.At first, the table is pushing back on the paper，and if you move the ruler quick enough，other air around the edges of the paper can’t get under the paper fast enough，so you are trying to lift that 9，300 pounds with the ruler! Some air gets under the paper，but not enough.so the ruler breaks.

　　By doing the experiment，we may realize

　　.

　　A.that air exits everywhere

　　B.why there are directions

　　C.that we live with air pressure

　　D.how air helps on the Earth

　　How many steps are there in this experiment?

　　A.2.

　　B.3.

　　C.4.

　　D.5

　　The underlined word”execs”probably means

　　.

　　A.covers completely

　　B.advocates actively

　　C.influences gradually

　　D.presses heavily

　　The ruler breaks under a quick strike mainly because of

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