Read the following text. Choose the best word(s) for each numbered blank and mark A, B, C or D on ANSWER SHEET 1. (10 points)3（）

A．from
B．in
C．by
D．on

Air is (1) , and like all matter, it has weight. Weight is the measure of the (2) of gravity (3) matter. If a scale registers 10 kilograms when a stone is placed on it, this means that gravity pulls the stone with that (4) force. (5) , earth’ s gravity pulls on each (6) of gas and dust on the atmosphere. Because our atmosphere is a. vast (7) of air, it has (8) . If it could (9) be compressed and put on a (10) of scales, it would weigh about 5,700,000,000,000,000 (quadrillion) metric tons.

The air pressed down on us and against us (11) all sides. Something (12) a ton of air is pressing against you at this moment. You are not (13) of this because air pressure within your body (14) the pressure of the air outside.

Air pressure is 1. 036 kilograms per square centimeter (14.7 pounds per square inch) at sea (15) . It is greatest there because that is the (16) of the atmosphere. (17) higher altitudes the pressure is (18) . That is why the (19) of highflying planes are pressurized. They are designed to (20) the air pressure our bodies must have.