A study of traffic violations on a busy highway finds that the number of tickets for speeding written by officers tends to decrease on rainy days. A police sergeant explains that driver behavior could be a confounding variable. Which statement best explains the confounding variable in this study?

On sunny days, more tickets for speeding are written than on rainy days.
Officers are less likely to want to leave their police cars to write tickets on rainy days.
Drivers may drive more carefully on rainy days, so the number of speeding tickets decreases.
Drivers are more likely to speed on rainy days to get to their destination faster.