In an experiment to study the effect of pH on frogs, how should a control group be established?

To effectively establish a control group in an experiment studying the effect of pH on frogs, it is essential to use a set of frogs that have similar characteristics while keeping certain variables constant. By taking frogs from specific habitats to test against those in environments with varying pH levels, researchers can observe the outcome of pH changes while controlling for other factors that might influence the results.

This method ensures that any observed effects on the frogs can be attributed specifically to changes in pH rather than other environmental factors, such as the frogs' origin or size differences. Establishing a baseline through a controlled group in this manner allows for meaningful comparisons that lead to more reliable and valid conclusions about how pH affects frogs.

The other choices do not adequately control variables. For example, using frogs from different locations could introduce genetic or environmental variations that might skew the results. Testing only one pH level would not provide any comparative data necessary to understand the effects of varying pH, and focusing solely on frogs of the same size may not address the broader ecological implications, which include the broader range of environmental adaptations that exist in amphibians.