Acontrolled experiment is repeated many times to ensure that the results are accurate includes at least two groups, one of which does not receive the experimental treatment includes at least two groups, one differing from the other by two or more variables includes one group for which the scientist controls all

includes at least two groups, one of which does not receive the experimental treatment  

Explanation:

In a scientific experiment, a controlled experiment is one in which two groups (two samples) are compared to test a hypothesis: the control group (also called the test or testimonial group) and the experimental group. The control group, which does not undergo any intervention, serves as a comparison element with the experimental group, in which some alteration is introduced. Since only one variable is tested in the experimental group, while the other conditions are maintained (at least in theory) strictly equal to those of the control group (reference), it will be possible to explain the differences eventually found at the end of the experiment between the two groups - validating or not the hypothesis tested.

That is, a controlled experiment is one that includes at least two groups, one of which does not receive the experimental treatment.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."