Answer:
control group
Step-by-step explanation:
In an experiment, the control group is one that does not receive the "factor" that is being tested in the experiment, and is maintained under normal circumstances.
In relation to the question above, we can see that an experiment was established with the objective of testing the results of exposing a group of bacteria to quantities of mercury greater than the quantities found naturally in the environment, that is, this experiment wanted to test the effect of large quantities of mercury in bacteria. For this reason, we can conclude that the group of bacteria that was subjected to normal amounts of mercury is the control group.