I believe the answer is A im not sure but to me it sounds right.
Aquatic is the answer because they domesticated eels and fish in early history
Idk the timeline but from largest to smallest, this hierarchy includes eons, eras, periods, epochs, and ages.
Answer - It is a Great Concern because microbes such as a hydra can consume some of that mercury which then a shrimp can consume that microbes which then transfer into the mouth of a trout which then when humans fish they consume the fish which later we get Mercury poisoning.
Reasoning - There are more concern but these are a few and some reasoning.