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

Pablo wanted to grow cucumbers from seeds. He read that using a heating pad to heat up potting soil could help make seeds germinate, or sprout, faster. Pablo wondered whether the temperature of the heating pad would affect how quickly the seeds germinated.
Pablo prepared two potting trays, each made up of ten small pots of soil. He planted one cucumber seed in each small pot and arranged the potting trays near a sunny window. He set an electric heating pad to 75°F and placed it under one potting tray. He set a second heating pad to 85°F and placed it under the other potting tray. Pablo observed the pots daily, and he counted the number of days it took until a seed germinated in each pot.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: germinating plants in a potting tray.
Which of the following was an independent variable in this experiment?
A. the number of days until a seed germinated
B. the temperature of the heating pad
From the given choices, select the correct answer and reply with the letter of the chosen option.

B