All over the earth"s surface is a layer of air which extends upwards for many miles. This air (1)_____ the oxygen without which neither plants nor animals (2)_____ live. Its movements, temperature and pressure (3)_____ the weather, and it is a vehicle (4)_____ the clouds of water vapor (5)_____ condense and fall as rain. It forms a blanket which (6)_____ us from the extreme heat of the sun during the day and (7)_____ the extreme cold when the sun has (8)_____ It is chiefly (9)_____ air that sound travels, so that if there were no air we should (10)_____ practically nothing. The atmosphere is held (11)_____ the earth"s surface by the gravitational pull of the earth—that is, it has weight. High up it is thin (12)_____ near the surface it is compressed by the (13)_____ of air above, and is more dense. The weight of air pressing (14)_____ each square inch of surface at sea-level is nearly (15)_____ pounds, which means that the total force (16)_____ the skin of an average man is about 30,000 pounds. He is not, (17)_____ this because the pressure is equal in all directions and the pressure inside him is equal (18)_____ -that without, but should he go up in a balloon to a height at which the outside- pressure is (19)_____ he would suffer acutely. It is (20)_____ this reason that the cabins of aeroplanes are "pressurized".

A. in
B. on
C. for
D. around