XDF SC 4-12

CasualWalk

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！

1stzhang

upward是副词，所以不能用extent

CasualWalk

I see. 谢谢。