The group in a experiment that is not exposed to the variable is called the

is not exposed to the independent variable under investigation and so provides a base line against which any changes can be compared

the answer to your question is b

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.