XDF SC 4-12

equilibrium

Most nations regard their airspace as extending upward as high as an aircraft can fly; no specific altitue, 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 肯定是a or b, 但我觉得a,b都不顺。答案为A。请指点一下extending在这里的用法， 谢谢！

huahua115

upward是副词，所以不能用extent

sammen

同意楼上，另外an aircraft can fly个人感觉是个动态的过程，在不断变化的。和extending正好匹配。

equilibrium

I see. 谢谢。