单项选择题

According to the passage, a complete conversion can take______.

A．several months
B．about a year
C．half a year
D．several years
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单项选择题
The two main requirements of aircraft design are______.
A．speed and cost
B．reliability and passenger comfort
C．lightness and dependability
D．ability to stay up in the air and reliability
单项选择题
The stress man's job is to______.
A．test how safe the plane is
B．calculate how strong each part must be
C．work out what height the plane may fly at
D．control the amount of luggage each passenger may carry
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