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By "gap" I presume you refer to the ellipses (.......)

"Because" suggests cause of something. "I got a hamburger because I was hungry" or "These birds migrate from Alaska to South America because during winter their food supplies in Alaska disappear".

In your example it might not be the case that a rise in tourism is caused by increased interest (or perhaps "increased interest" isn't specific enough as an explanation). It could be, for example, a local drop in the cost of rental properties or a new amusement park, etc.

"Because" can be translated to mean "and the reason is that."

There has been more interest "and the reason is that" the number of tourists to the area has risen...

Or

The number of tourists to the area has risen "and the reason is that" there has been more interest.

The number of tourists to the area has risen BECAUSE there has been more interest.

Or you might mean (and I do not know).

There has been more interest in this geographic location "and the evidence for that is the fact that" the number of tourists to the area has risen.

If the above is more your meaning, then you could say

There has been more interest in this geographic location, and I know that because the number of tourists to the area has risen.

I hope that helps.

Thanks a lot guys! I get it already!

Just FYI: The correct answer in the gap should be: "Although/While