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

Every day at work, Colleen poured a large cup of tea that she drank throughout the morning. She wondered how much bacteria might be growing in her cup of tea as the morning went on.
To find out, Colleen filled three identical cups with tea and placed them on her desk for three hours. After the first hour, she placed a drop of the tea from each cup on a separate microscope slide and counted the number of bacteria in the drop. Colleen repeated this with each cup of tea after the second hour, and again after the third hour.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: using a microscope to count bacteria on a slide.
Which of the following was an independent variable in this experiment?
A. the amount of time that the tea sat on the desk
B. the number of bacteria in a drop of tea
Identify the correct option from the choices provided and respond with the letter of the correct option.

A