The black band disease is caused by the tissue degrading action of cyanobacteria on the skeleton of the corals. Cyanobacteria thrive in places that are rich in nutrients. Hence the coral reef B will be the most affected one, as the water there will have a nutrients from the farm run-offs, that will help the cyanobacteria thrive in large numbers to deteriorate the condition of the exposed star corals. Reef A will comparatively less affected to reef B as the water is clean and is not contaminated by any run-offs to increase the water nutrient content.