In this clip, you can hear the following question:
Recent polls have shown a fifth of Americans can’t locate the US on a world map. Why do you think this is?
It’s not clear to me if the clause “This is” leaves out something, e.g. “This is like that”, “This is true” and so on, or if, on the contrary, the verb “to be” is used here with a full stative meaning, as if it meant “This exists“.
Moreover, is this usage of the verb “to be” linked to questions such as that I gave as an example, or can it also be used in affirmative sentences?