Answer: Experimental studies directly influence something to see its effect, but observational studies do not directly influence the subjects.
Explanation:
The correct option is D.
In an experimental study, you perform an experiment. In an experiment are made with the objective of validating, refute, or support a given hypothesis, where you observe "what happens" when you modify a given set of parameters. Always in an experiment, we can define the independent and dependent variables, where the independent are the ones that we can modify at will, and the dependent ones are the ones that we observe to see how they react to the changes in the independent variables.
In observational studies, the idea is not changing anything in the object of study, so the scientist in this case are only observers or spectators.