Context: The passage below describes an experiment.

The  in Devon's lunch always turned brown by lunchtime. Devon's favorite cooking website said that pouring lemon juice on the  would keep them from turning brown so quickly.
Devon 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, Devon counted the number of  that had turned brown.
Figure: apple slices.
In this experiment, which were part of a control group?
A. the plain apple slices
B. the apple slices covered with lemon juice
Identify the correct choice from the options below and respond with the letter of the correct option.

A