题目内容：
回答题：Transportation
For many years in the desert, camels Used to be the only form of transportation (运输)．Before the(51) of modern trains, camel trains used to carry all the goods for trading between Central Africa and Europe．Traders sometimes (52) to put together camel trains with 10,000 to 15,000 animals．Each animal often carried (53) 400 pounds and could travel twenty miles a day．This form of transportation was so important that camels were called the "ships of the (54)."
Now modern trains travel across the desert in a very (55) time．One engine can pull as much weight as 135,000 (56)．In addition, trains use special cars for their load．Refrigerator cars carry food; boxcars carry heavy goods; stock cars carry animals; and tank cars carry oil.
Air travel has changed, too．The earliest planes were biplanes (双翼飞机), with(57) sets of wings．The top speed of this plane was 60 miles per hour．The pilots used to sit or lie on the wings in the open air．The plane(58) sometimes stopped in the middle of a trip．It used to be (59) to fly in bad weather．In snow or in rain, the wings frequently became icy．Then the plane might go down.
Mechanical improvements during the First World War changed airplanes．Monoplanes (单翼飞机) took the(60) of biplanes．Pilots flew inside of covered cabins．Still, even these planes were small and expensive．Only(61) people were be able to travel in airplanes.
Now modern jets make air travel possible for all people．No place in the world is more than 24 hours away by jet．Further improvements have (62) the cost of flying, and they have made air travel (63) safer than it used to be．
A modern 707 can carry 170 people and can fly at 600 miles per hour．People(64) used to eat, sleep, or watch movies on airplanes.(65) these things are a normal part of air travel!
A．age
B．series
C．year
D．period
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