请教nn一道语法题...谢谢:)

vict2006

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

Eto extend  

B为什么不行?

abort

http://forum.chasedream.com/dispbbs.asp?boardID=23&ID=22920看看前辈的讨论