Context: The passage below describes an experiment. Read the passage and think about the variables that are described.

Carson's daughter complained that the  in her lunchbox turned brown by lunchtime. Carson read that when  are exposed to air, oxygen in the air reacts with the cut surface of the , turning them brown. Carson's favorite cooking website said that pouring lemon juice on the  would slow down this chemical reaction.
Carson cut an apple into ten slices. He put five of the slices into a bowl and poured two tablespoons of lemon juice over them. He put the other five slices into another bowl and did not add any lemon juice to them. After one hour, Carson counted the number of  in each bowl that had turned brown.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: apple slices.
Which of the following was an independent variable in this experiment?
A. the amount of lemon juice added to the apple slices
B. the number of apple slices that turned brown
From the given choices, select the correct answer and reply with the letter of the chosen option.

A