Most nations regard their airspace as extending upward as high as an aircraft can fly; no specific altitue, however, has been officially recognized as a boundary. a) as extending b) as the extent c) to be an extent d) to be an extension e) to extend 肯定是a or b, 但我觉得a,b都不顺。答案为A。请指点一下extending在这里的用法， 谢谢！

I see. 谢谢。