Question

the main advantage of experiments over observational studies is that
a well - designed experiment can give good evidence that the treatment actually causes the response.
an experiment can compare two or more groups.
an experiment is always cheaper.
an experiment is always shorter.
we can include more than one explanatory variable in the study.

Explanation:

Observational studies only observe existing conditions and cannot establish causal relationships, as confounding variables may explain correlations. Well-designed experiments use random assignment to control for confounding variables, allowing researchers to attribute changes in the response variable directly to the treatment, providing evidence of causation. The other options are incorrect: observational studies can also compare groups or include multiple explanatory variables, and experiments are not always cheaper or shorter.

Answer:

a. a well-designed experiment can give good evidence that the treatment actually causes the response.