Pankaj0901
How do we know that "upward as high as an aircraft can fly" is an adverbial phrase and not an "adjective". I thought "upward" can act as adjective, and so can modify a "noun", ie "extent".
Example: The upward trend in revenue this year looks promising.
=> "upward", which is adjective, modifies "trend", which is a noun.
Similarly, in the given sentence, "upward" should be able to modify the noun "extent", and the phrase "as high as an aircraft can fly" should modify "upward".
What am I missing here?
There are a couple of issues with this. First of all, a nation's airspace isn't, itself, an "extent". You could refer to the extent
of the airspace, but the airspace does not equal the extent. In other words, the "extent" is a quality or characteristic of the airspace -- it isn't the airspace itself.
Second, even though something like a trend can in fact be upward, it doesn't make sense for an extent to be upward. Again, the extent is a quality of the airspace -- the area or space that it includes. The
area of the airspace it isn't moving in an upward direction -- the area of the airspace is fixed (right above the nation itself), and that airspace isn't really moving, changing, or growing (barring any border changes).
Similarly, you wouldn't say that the extent of a skyscraper IS upward; instead, you would say that the skyscraper EXTENDS upward. By the same token, you could describe a skyscraper as EXTENDING upward -- and that's pretty much what we have in (A).
I hope that helps!