补充 99

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...