Which of the following is related to case control study

a. Are used very commonly in environmental epidemiologic research. An example may be possible health effects of exposure to electromagnetic fields.

b. The units of analysis are groups. This can be referred to as an ecological comparison study.

c. Allowance of the direct observation of risk. The exposure factor is well defined and can study exposures that are uncommon in the population. The temporal relationship between factor and outcome is known.

d. None of the above.