Subjects in experimental and control groups should have matching skills. true or false

False, the subjects in experimental and control groups are same. But the control do not undergo any experimental test.  

Explanation:

In a scientific experiment, we take 2 objects for experiment one is an experiment and one is control.  In experiment animal, we apply many tests on it to know the result of the experiment. But in control animals, there is no experiment on it. one keeps it in its normal condition to compare with the experimental animals.

E.g. The thyroxine hormone increases the metamorphosis of a toad.

It is known by comparing the 2 types of tadpole i.e. one having thyroxine injected on it, and other is normal.  

The control is an independent variable which is compared to the experiment variable/animal to know about the effect of the test.

Therefore, the experimental skill is applied to the experimental object not to the control one.

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