Answer: The number of collisions.
An object, a time period, an event, an idea or any other category a person is intended to measure is known as a variable. There are two types of variables: the independent and the dependent. An independent variable is a variable that doesn't depend on other factors and doesn't change by other variables you're trying to measure. On the other hand, a dependent variable is related to another factors and changes accordingly. In this case, the dependent variable is the number of collisions.