517. Most nations regard their airspace as extending upward as high as an aircraft can fly; no specific altitude, 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（A）
(E) to extend
why not B? regard sth. as sth. airspace 和extent都是名词，为什么用extending?谢谢。 | 
