Do you mean "why"?
They are the main source for all living organisms because they provide energy, relatively short term, for any creature that consumes them.
D) allele frequencies of some traits change in the population
Alleles can be phenotype (how they appear) and natural selection is when an organism changes in a way that helps them survive.
It can be the way their beaks are formed that gives them a better chance of catching food among other stuff (Darwin's investigation)
Or how they change their color to adapt to the sudden change in tree coloration (Pepper moths)
Answer: c. Lawn fertilizer that washes into neighborhood ponds likely causes algae to become more abundant.
Explanation:
Eutrophication is a natural phenomena in which the algal growth occurs in the water body due to increase in concentration of the fertilizers and manures being drained from the agricultural land into water. This causes decrease in oxygen level in water, and the aquatic animals in water will not survive due to scarcity of water.
According to the given situation, the growth of algae in the neighboring ponds is due to the drainage of lawn fertilizers into the neighboring ponds being washed from the lawns. This causes algae to grow abundantly in ponds.
