Answer:
A.
Explanation:
In any experimental design, it is needed to have different groups or set of subjects, exposed to the treatment that is being studied, the experimental group.
But, since it is required to compare two groups, prove a hypothesis and conclude if the used treatment works or not, there is also a need to have another set of subjects not exposed to the treatment, control.
In this particular experiment, the experimental group is the group of linches grown under visible light and UV light, and the control group is the group of lichens grown in visible light without the UV light.
The hypothesis is whether or not ultraviolet light affected the growth of lichens, the dependent variable is the growth of the lichens and the limiting factor is an external agent that the researcher can not control.
