肩关节由（）和（）构成。

An Experiment
Materials: Newspaper, ‘ACE’ hardware ruler (1/8 “think”), a flat table.
Purposes: We’ll show that there is air pressure pushing on us, from every direction while we’re on this Earth.
Procedure:

First put a thin ruler on a flat table with a little
less than half of it hanging off the edge of the table
(see the drawing below). Next place a sheet of newspaper
over the ruler flat against the table. Make sure to have
as little air as possible under the paper so that the fold line of the newspaper is at the ruler. Then quickly strike the end of the ruler hanging off the edge of the table. If you strike it quick enough, the ruler will break near the table edge.
What’s going on?
The Earth is covered in a layer of air that is nearly 80 miles thick and at sea level (the bottom) 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.
1．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
2．How many steps are there in this experiment?          .
A．2      B．3       C．4       D．5
3．The underlined word “exert” probably means       .
A．cover completely            B．advocate actively    
C．influence gradually  D．press heavily
4．The ruler breaks under a quick strike mainly because of          .
A．the air pressure on the newspaper   B．the heavy weight of the newspaper
C．the heavy weight of the flat table    D．the strong power used on the ruler