英语论文网

17.The knives referred to in line 27 are used to
(A) separate double-stranded wire
(B) prevent the reel from advancing too rapidly
(C) twist the wire
(D) cut the wire that becomes barbs

18.What is the author s purpose in the third paragraph?
(A) To explain the importance of the wire.
(B) To outline the difficulty of making the wire
(C) To describe how the wire is made
(D) To suggest several different uses of the wire.

19.According to the passage, concertina wire is used for
(A) livestock management
(B) international communications
(C) prison enclosures
(D) military purposes.

20.Which of the following most closely resembles the fencing described in the passage?
(A) （图）
(B) （图）
(C) （图）
(D) （图）

Question 21-29

Under certain circumstance the human body must cope
with gases at greater-than-normal atmospheric pressure. For
example, gas pressures increase rapidly during a dive made
with scuba gear because the breathing equipment allows divers
to stay underwater longer and dive deeper. The pressure
exerted on the human body increases by 1 atmosphere for every
10 meters of depth in seawater, so that at 30 meters in seawater
a diver is exposed to a pressure of about 4 atmospheres.
The pressure of the gases being breathed must equal the
external pressure applied to the body; otherwise breathing is very
difficult. Therefore all of the gases in the air breathed by a
scuba diver at 40 meters are present at five times their usual
pressure. Nitrogen which composes 80 percent of the air we
breathe usually causes a balmy feeling of well-being at this
pressure. At a depth of 5 atmospheres nitrogen causes symp-
toms resembling alcohol intoxication known as nitrogen narcosis.
Nitrogen narcosis apparently results from a direct effect
on the brain of the large amounts of nitrogen dissolved in the
blood. Deep dives are less dangerous if helium is substituted
for nitrogen, because under these pressures helium does not
exert a similar narcotic effect.

As a scuba diver descends, the pressure of nitrogen in the
lungs increases. Nitrogen then diffuses from the lungs to the
blood and from the blood to body tissues. The reverse occurs
when the diver surfaces; the nitrogen pressure in the lungs
falls and the nitrogen diffuses from the tissues into the blood
and from the blood into the lungs. If the return to the surface
is too rapid, nitrogen in the tissues and blood cannot diffuse
out rapidly enough and nitrogen bubbles are formed . They can
cause severe pains, particularly around the joints.

Another complication may result if the breath is held dur-
ing ascent. During ascent from a depth of 10 meters, the
volume of air in the lungs will double because the air pressure at
the surface is only half of what it was at 10 meters. This
change in volume may cause the lungs to distend and even rup-
ture. This condition is called air embolism. To avoid this
event, a diver must ascent slowly, never at a rate exceeding
the rise of the exhaled air bubbles, and must exhale during
ascent.

21.What does本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。