Manure is carried by rain water from land to a river. this is an example of

single pollution

multiple pollution

point source pollution

non-point source pollution

non-point source pollution  

Explanation:

A non-point source pollution is the one which originates from many sources and the origin of the sources cannot be traced back. As a result this kind of pollution cannot be controlled. A point source pollution is the one which can be traced back to it's source pollutant and a single pollutant is responsible for the pollution. It can be controlled.

The given situation is an example of non-point source pollution. As the source of manure cannot be traced back to it's origin because rainwater caused the surface runoff of the manure which get mixed with the water of river.

measuring light intensity

light meters can be used to measure light intensity. the meter is held at the soil surface and pointed in the direction of the maximum light intensity, and then the meter is read.

errors can be made when measuring light intensity by accidentally shading the light meter. the reliability of the results can be checked by taking many samples.

bacterium is not correct