Jeffreys, a research, is conducting a drug trial in a controlled environment. He manipulates several variables and measures the effect of each variable on the subjects of the trial. In this scenario, Dr. Jeffreys is using the method.

experiment

Explanation:

Experiment method: In research, the term experiment method is described as a method of investigation whereby a specific hypothesis is being scientifically tested. Therefore, in the experiment method, a dependent variable is being tested or measured by the researcher, an independent variable is being changed or manipulated, and any extraneous variables are being controlled by the experimenter, investigator or researcher.

Advantage: The experiment method is objective in nature.

In the question above, Dr. Jeffreys is using the experimental method.

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industry had made the u.s. the richest nation in the world at this time. the second industrial revolution caused growth in industry and transportation, which allowed increased trade between nations.