单项选择题

The noise-killing system is net yet popular mainly because______.

A．it is still being tried out
B．drivers still have their doubts
C．it increases the cost of car production
D．carmakers are not sure if it is necessary
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单项选择题
Under its Charter, the U.N. guarantees (保证)______.
A．never to use arms
B．to employ international machines
C．better standards of life
D．to promote economic and social advancement
单项选择题
A member country cannot ally (结盟) itself with ______.
A．other U. N. member countries
B．other countries not members of the U. N.
C．countries resisting openly the U. N.
D．states against which the U.N. has ever taken preventive or enforcement action
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