Question

: Did you know that ______Colfax Avenue is _the__ longest continuous street in _the_ United States?

Answer 1

the answer is Denver's Colfax Avenue

Answer 2

Did you know that [nothing] Colfax Avenue is the longest continuous street in the United States.

The should always be used with United States. Also, Many the in a sentence looks awkward so we'll just use 2 here.