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！ | 
