If the distance between two objects is increased, the gravitational attraction between then will either increase, decrease, or remain the same

So as the mass of either object increases, the force of gravitational attraction between them also increases. ... Since gravitational force is inversely proportional to the square of the separation distance between the two interacting objects, more separation distance will result in weaker gravitational forces.

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

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.