The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

We are successful because we can form long—lasting relationships with many others in diverse and flexible ways, and that this, combined with our native intelligence, explains why Homo sapiens came to dominate the planet. In every way from teaching our young to the industrial division of labor we are a massively co—operative species that relies on larger and more diverse networks of relationships than any other species.
（）of relationships.