What is the limiting factor in oligotrophic lakes?

Populations of algae and the animals that feed on them are lower in oligotrophic lakes because of low nutrient concentrations. Thus the water remains clear. The decay of the relatively small amount of organic matter in oligotrophic lakes does not completely deplete the hypolimnetic supply of dissolved oxygen. Therefore, lack of oxygen does not restrict animals from living in the hypolimnion of oligotrophic lakes. Lake trout, for example, require cold, well-oxygenated water and primarily live in the hypolimnion of oligotrophic lakes. Minnesota's oligotrophic lakes are found in the northeast region of the state, where infertile soils are covered with mixed conifer forests.

Mg is used to measure mass

The right answer is c.sandstone.