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

Dr. Spencer was the land manager for a prairie that had become overrun by unwanted grasses. These grasses crowded out other plants. Dr. Spencer thought that she could use fire to remove the unwanted grasses and allow other plants to grow. But she didn't know how often the prairie should be burned.
Dr. Spencer marked off six plots within a large area of the prairie. She used carefully controlled fires to burn all of the plants in each plot. She burned three of the plots once a year for three years. She burned the other three plots twice a year for three years. A year after the last fire, Dr. Spencer counted the number of unwanted grasses in each of the six plots.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: a fire burning through a prairie.
Which of the following was an independent variable in this experiment?
A. the number of times the plots were burned each year
B. the number of unwanted grasses
Identify the correct option from the choices provided and respond with the letter of the correct option.
Answer: A