Answer:
The control group will be the houses without apples.
A control group is set up in an experiment in other to be used to compare the outcome of the experimental group. The control group are subjects whose variables aren't changed throughout the cause of the study. This means that the control group are not given any treatment.
In the experiment above, The independent variable is the treatment added to our experiment, which are apples.
Hence, since the control group are not given the treatment, then the control group will be the houses without apples and are thus unchanged.