15. The main idea of the sixth paragraph (the highlighted phrase) is that the:
Your Answer is
Correct Answer is B
Explanation
Lines 40-46 have two layers of information, with in addition connection in the middle. The first layer of information lines 40-43: the surveys produced geographic revelations; the second layer of information: Bidlack Treaty of 1846 was the basis of involvement of US. So the general idea of this paragraph is that the combination of the two corresponds to B.