Epidemiologic studies that examine the role of a suspected factor in the etiology of a disease may be observational or experimental. The most important difference between experimental and observational studies is that in experimental studies:

a. The study and control groups are equal in size.
b. The study is prospective.
c. The study and control groups are always comparable with respect to all factors other than the exposure.
d. The investigator determines who shall be exposed to the suspected factor and who shall not.
e. Controls are used.

the correct option is D (The investigator determines who shall be exposed to the suspected factor and who shall not.)

Explanation:

Epidemiology is the branch of medicine that deals with the study of diseases in a defined population to obtain evidence that will aid in the prevention, treatment and control. It is divided into 2 categories:

- experimental studies and

- Observational studies.

In Observational studies, the epidemiologist collects data of interest based on what is seen or observed from the exposure or disease status of each study participants. While in experimental studies, the investigator determines through a controlled process the exposure for each individual and then track over time to detect the effects of the exposure. Therefore in experimental studies,the investigator determines who shall be exposed to the suspected factor and who shall not. I hope this helps, thanks!

producers within ecosystems are essential as they are at the base of the food chain. plants are examples of producers. they use the process of photosynthesis to capture light energy and use it to convert simple inorganic substances (carbon dioxide and water) into organic substances (glucose).