If the control lake in a coal plant study is actually a geothermal hot spring with acidic water, how might this affect the results?

• A. The coal-burning plant would appear to have an unusually large impact.
• B. The coal-burning plant would show a variable, unpredictable impact.
• C. The coal-burning power plant would not appear to have a large impact because the pH levels of the two lakes would be similar.
• D. The geothermal lake would neutralize the coal plant's impact.

Answer: (C)

The main idea here is that a control should provide a true baseline that matches the treatment in all relevant ways except for the factor being tested. In an aquatic study of acid pollution, pH is a key measure of acid deposition. If the control lake is actually a geothermal hot spring that already has acidic water, it already sits at a low pH comparable to what the coal plant might cause. When you compare the treatment lake (exposed to the coal plant) to that control, there isn’t a clear baseline difference to attribute any additional acidity to the plant. The two lakes end up with similar pH levels, so the coal-burning plant’s impact would not appear large.

So, the best answer is that the coal-burning power plant would not appear to have a large impact because the pH levels of the two lakes would be similar. This highlights why proper controls need to resemble the untreated condition closely before the variable is applied.

The other ideas don’t fit as well because a true baseline that’s already acidic would not exaggerate the plant’s effect, and a different control scenario would be needed to reveal the plant’s influence.