Answer: A. their base level
The base level is the lowest level in which running water can flow and is capable of eroding. The base level of a stream is the point where the running water has less velocity, which leads to decreased speed for the running water and reduced or zero erosion. A typical example of a base level is a lake into which a stream flows. The velocity of the running water from the stream reduces greatly as it comes in contact with a bigger water body.