Biology

The constant variable in this study would be:

Speed of the car.

A constant variable is something that is kept the same throughout the experiment to ensure that any observed effects are due to the independent variable, not other factors. In this case, the study is testing how different tire treads affect the braking distance of a car. To make sure the test is fair, the speed of the car should remain the same for all trials, as different speeds would affect braking distance.

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