弱爆了，大家都没有疑问的地方我却不懂！求解啊，大恩不言谢！

大宝店小二

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
E为什么不能选呢，upward不能修饰动词extend吗？

jeffery2541

答案是A？