Base level a) the elevation of a streams largest tributary b) the source from which the streams flow c) the lowest point to which a stream can erode its chanel d) any part of the stream below sea level

is this for the e2020 class

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.