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

Giardia is a microscopic parasite that lives in water and can infect humans. Dr. Hoffman designed a drinking straw that contained a filter to remove Giardia from water. Dr. Hoffman wanted to know if a longer filtering straw would remove more Giardia.
Dr. Hoffman made six filtering straws: three that were five inches long and three that were ten inches long. She prepared six one-liter batches of water, each containing 10,000 Giardia. Then, Dr. Hoffman passed one batch of water through each straw. After each batch passed through the straw, she used a microscope to count the number of Giardia that remained in a small sample of the water.
Hint: An independent variable is a variable whose effect you are investigating. A dependent variable is a variable that you measure.
Figure: the parasite Giardia, viewed with a microscope.
Which of the following was an independent variable in this experiment?
A. the length of the filtering straw
B. the number of Giardia that remained in the water
Identify the correct option from the choices provided and respond with the letter of the correct option.

A