请教regard 的用法

12.   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

是的，编辑一下吧：）