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. forms
B. contains
C. consists
D. fills