Denise wants to find out how the steepness of a hill’s slope affects the amount of soil that erodes from it. She will use soil, a long pan, and several 4-inch-tall wooden blocks to build a model hillside. She will use a watering can to simulate rainfall. In her experiment, which factor should be Denise’s variable?

the type of soil with which she fills the pan
the number of blocks holding up the end of the pan
the amount of water she pours on the model hillside
the location she uses to test her model