Hillary is dropping objects that are the same volume. She wants to investigate how an object’s mass affects the average speed at which it travels when dropped, until it hits the ground. Which variable must Hillary change during the experiment? A. the timer
B. the drop height
C. the mass of the object
D. the place where she drops it

This is what i found on google “according to the collision theory, the following criteria must be met in order for a chemical reaction to occur: molecules must collide with sufficient energy, known as the activation energy, so that chemical bonds can break. molecules must collide with the proper orientation.” i hope this information you.