Read the following passage and choose the best answer (A, B, C, D):
“Evidence has piled up to show that our relationships, including friendships, affect  our health at a much deeper level, tweaking not just our psychology and motivation but the function and structure of our organs and cells,” writes science journalist Lydia Denworth in the book “Friendship: The Evolution, Biology, and Extraordinary Power of Life’s Fundamental Bond” (W.W. Norton), out now. The average American claims to have about four close friends, and the great majority of us have between two and six. Only 5 percent of those studied had more than eight, while 5 percent had no close friends.  Still, 20 percent of us call ourselves lonely — and the health implications are overwhelming. We’ve known about the fallout of loneliness since a 1988 Science paper concluded that being lonely “constitute[s] a major risk factor for health” equivalent to obesity, smoking and lack of physical exercise. According to Denworth’s research, lack of social contact in the elderly costs Medicare $6.7 billion a year. There are purely positive relationships, which make up about half of our social networks, and then purely negative ones, which are rare. And then there are the uncomfortably ambivalent ones that land in-between — aka “frenemies.” These ambivalent bonds make up about half of our social networks, but the research on them is pretty stark: “Ambivalent relationships are bad for us,” writes Denworth. These relationships are reportedly causing levels of inflammation, aging, blood pressure and even greater artery calcification. This is true for even not so intimate relationships, such as colleagues and neighbors, too. Sadly, about 50 percent of married view their spouses ambivalently, according to Brigham Young University researchers. According to the book, it takes between 40 and 60 hours to create a casual friendship and over 200 hours to become a “best friend.”
1. What is main topic of the passage?

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