Context: The passage below describes an experiment.

The  in Tammy's lunch always turned brown by lunchtime. Tammy's favorite cooking website said that pouring lemon juice on the  would keep them from turning brown so quickly.
Tammy cut an apple into eight slices. She poured two tablespoons of lemon juice over four of the slices. She did not pour lemon juice over the other four. Every hour for three hours, Tammy counted the number of  that had turned brown.
Figure: apple slices.
In this experiment, which were part of an experimental group?
A. the apple slices covered with lemon juice
B. the plain apple slices
Pick the correct answer from the listed options and provide the letter of the selected option.

A