A home-improvement store offers a new brand of lawn food that claims to reduce the number of weeds by 30% over a different brand of lawn food. To test the claim, a homeowner plans to use the new brand of lawn food on half of his lawn and the original brand on the other half. The homeowner will count the number of weeds that appear after a month on each half of the lawn. A neighbor explains that there may be confounding variables, such as shade, that might also explain the relationship. Which statement best describes the effect of the confounding variables?

Because of confounding variables, the homeowner should only put the new lawn food on shady spots and the original brand on sunny spots.
Because shade is a primary cause of weed growth, the brand of lawn food used in shady or sunny spots will not affect the number of weeds.
If the half of the lawn with the new lawn food shows fewer weeds after a month, this will prove the new lawn food is better at stopping weed growth.
The parts of the lawn that receive the new lawn food may also get more shade than other parts of the lawn, and the shade might also cause fewer weeds.