Marcelino is setting up five aquatic environments with different mercury concentrations, one of which has 0 ng/L of mercury. This environment is not exposed to the variable being tested (mercury). In an experiment, the setup where the independent variable (here, mercury) is not applied is called the control. The control serves as a baseline or standard for comparison to see the effects of the variable being tested. Let's review the answer options: - test experiment - Usually refers to the setup where the variable is applied. - moderating experiment - Not a standard term in experimental design. - null experiment - Usually refers to an experiment done under conditions where no effect is expected, but not the standard term for the baseline in this context. - control experiment - This is used as the baseline or standard to compare the effects of the independent variable