[求助]大全517

1.        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

答案是a,但我觉得A也有不妥,regard sth as adj 这种用法好象不可以吧

谢谢指教