517. 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

why not B? regard sth. as sth. airspace 和extent都是名词，为什么用extending?谢谢

这个要从句意来理解. extending:延伸; extent:程度.

如果用extent, 意思解释不通.

 我觉得在这里，extending是动名词，有表示动作的概念，所以用extending好过

extend n.

我觉的关键的一个词还是upward，表明了一种向上的趋势，所以与动名词连接效果最好。

原来是动名词啊

但是，我有还有个问题  regard their airspace as extending upward什么意思 

regard sth1 as sth2 是认为某物1是某物2，那选项A中是怎么解释的呢