D. All of the above are correct
Algae and Cyanobacteria could mean life is flourishing and is a good food source for some fish in moderation- but if there's a lot of fertilizer runoff, they can grow too quickly and be counterproductive for the environment
The correct answer is frequency.
The pitch can be defined as the quality of sound produced by the rate of vibration producing it.
It is directly related to the the frequency of the sound wave. If the frequency is increased then its pitch will also increase and vice versa.
As for normal people it is a bit difficult to differentiate between the two but the musician are trained so they can differentiate the difference between the two even if the difference is 2 Hz
Law would be the correct answer to fill in the blank. Law is correct because scientific law explains behaviors of natural phenomenons since it's based on repeated experiments.