补充 99 Most nations regard their airspace as extending upward as high as an aircrafe can fly; no specific altitude,however ,has been offically recognized as a boundary
a) as extending
b) as the extent
c) to be an extent
d) to be an extension
e) to extend
the best answer is b
why not a ,because a emphasis the action and b emphasis the effect ,According the sentence I choice a .Help...
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