Regina conducted an investigation to determine whether earthworms add nutrients to soil. She used two pots of soil. She added three earthworms to the first pot. She did not add any earthworms to the second pot, which was the control. Regina tested the nutrient levels of the soil in both pots after two weeks.What type of investigation is Regina conducting?

Regina deliberately changed one condition by adding earthworms to one pot, while leaving the other pot without earthworms as a control. She then measured the soil nutrient levels to see whether that change made a difference. An investigation in which a researcher changes a variable and measures the result is experimental, so the answer is experimental.

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