Recent Studies Suggest That Flame Retardants, Chemicals That Are Used

Recent studies suggest that flame retardants, chemicals that are used to slow down burning, can mimic estrogen hormones found in the human body. Scientists investigating this phenomenon developed three-dimensional images to show how the chemical structure of certain flame retardants allows the retardants to bind to and inhibit an enzyme (estrogen sulfotransferase) that metabolizes estrogen.
Suppose the scientists want to conduct a study of the effects of flame retardants on estrogen levels. In a hypothetical study, the scientists use frogs to test the effects of flame retardants because the frogs' skin is highly permeable and therefore allows for easy absorption of chemicals.
The scientists collected 20 wild adult frogs and brought them to a laboratory. Ten of the frogs (group 1: five male and five female) were kept in tanks mimicking their natural environment. The other ten frogs (group 2: five male, five female) were kept in tanks like those of group 1 except they were introduced to low amounts of flame retardants in the shallow water in their tank. Estrogen levels were measured on the first day of the experiment and then every 4 days for 20 days.
-Why is group 1 needed in this experiment?

A) Group 1 is the standard of comparison for the experimental group.
B) Group 1 is the standard of comparison for only adult males.
C) Group 1 is the standard of comparison for only adult females.
D) Group 1 allows for reliable comparison of all ages of female frogs in the study.