How are primary and secondary succession similar and how are they different

Primary succession occurs following an opening of a pristine habitat, for example, a lava flow, an area left from retreated glacier, or abandoned strip mine. In contrast, secondary succession is a response to a disturbance, for example, forest fire, tsunami, flood, or an abandoned field.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."