单项选择题

A．Although
B．as
C．but
D．while
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单项选择题
A．even ifB．if onlyC．only ifD．as if
A．even if
B．if only
C．only if
D．as if
单项选择题
Under what conditions are helicopters found to be absolutely essential
A．For overseas passenger transportation.
B．For extremely high altitude flights.
C．For high-speed transportation.
D．For urgent mission to places inaccessible to other kinds of craft.
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