Question

the passage below describes an experiment.
donald wanted to grow moss for his terrarium. he usually grew moss on a bed of dirt but wondered what he could add to the dirt to make his moss grow faster. donald prepared nine moss cultures. in three cultures he mixed moss with only dirt. in three others he mixed moss with dirt and buttermilk. in the final three he mixed moss with dirt and used coffee grounds. he observed how much each moss culture had grown after two weeks.
in this experiment, which were part of an experimental group?
the moss cultures mixed with only dirt
the moss cultures mixed with dirt and buttermilk

Explanation:

In an experiment, the experimental group is the one where the independent variable is manipulated (here, adding buttermilk or coffee grounds to dirt). The control group is the one with only dirt (no manipulation). So the moss cultures with dirt and buttermilk (and also those with dirt and coffee grounds) are experimental groups as they have the manipulated variable (addition of buttermilk/coffee grounds) to test moss growth. The option "the moss cultures mixed with dirt and buttermilk" is part of the experimental group.

Answer:

the moss cultures mixed with dirt and buttermilk