Answer : The number of blocks holding up the end of the pan
Explanation :
As from the given information Nancy's aim is to find the steepness of a hill's slope which will affect the amount of soil that erode from the surface so, the variable that should not be changed in this experiment should be the height of the long pan. The height of the long pan is made using the blocks of books on which the end of the pan is balanced. This should remain uniform and constant throughout the experiment so that other variables of the experiment can be altered and the results can be verified accordingly.