extent用法

Most nations reagrd 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 乱搭

d. to be an extension 这个搞笑了点

e. to extend 乱搭

A B当中选哪个？我不同意书面答案，没有参考价值。

选A，这儿high是副词，要修饰动词短语extend(意思是Increasing,stretching),extent是程度，范围的意思。