885-04-12

请问885-04-12&23
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
(E) to extend
这题划线的UPWARD不在A里, 我想问,B选项为什么不对? 原文划线部分怎么理解才好呢?

extingding应该是adj吧，什么叫regard sth as adj as adv as....

请问这句话是什么意思啊谢谢

extending是动名词

大多数国家把领空向上延伸,高到飞机能够飞行的高度.