The more probable answer is b: power plant effluent; golf course runoff, although a: <span>power plant effluent; an underground coal mine drain, is also a possible answer.
The source of effluent from a power plant occupies a small, well defined space, i.e. an effluent pipe. This is what is known as a point source, as you can identify with high confidence the spatial and temporal boundaries of the source of pollution. Point sources are relatively easy to quantify and manage because they are so well defined in space and time. Reducing pollution from point sources could be achieved by simply treating effluent to a higher standard or reducing the amount of effluent released. Runoff from land, such as a golf course, is what is defined as a non-point source pollutant. The spatial and temporal aspects of this sort of pollution are much harder to define, as the pathways of runoff from the land into the river may be highly dispersed and variable over time. These sorts of pollution are therefore much harder to quantify and manage.</span>
<span> limiting factor </span>
<span>A limiting factor is anything that is in short supply in the environment and therefore limits life processes</span>
