Option B

“Unfortunately for coral reefs, however, a variety of human activities are causing worldwide degradation of shallow marine habitats by adding nutrients to the water….” The author’s views are clear in the above lines where he tries to cite out the human activities which have resulted to be a threat for the corals as slash-and-burn land clearing and sewage disposal. Although the author mentions how coral reefs produce food for themselves, that is not the central idea and hence option D is wrong. The author does not discuss deterioration about algae, option A is eliminated. Coral reefs do not deteriorate in nutrient poor waters. Option C is incorrect.