<span>B. Social interaction. Highly populated areas bring together people from all walks of life and due to circumstance these people end up relating with each other on one level or another. In most cases people living in highly populated areas will end up becoming friends over a long period of time as it is likely that members of their families may end up going to the same schools or work in the same areas.</span>
Among the given choices, the only positive effect that may be thought off with the increase in level of CO2 in the atmosphere is the increase of plant growth. Carbon dioxide is an essential compound in the photosynthesis of plants. Therefore, the answer should be letter C.
d is correct answer of a question