Carl knows that water moves through different kinds of soil at different rates. How easily water moves through a soil is known as permeability. Carl decides to compare the permeabilities of different soil types.

To do this, Carl takes five identical flower pots with holes in the bottom and fills each one with different soil:
rocky, gravely dirt from the side of the road
potting soil from the store
clay soil from a nearby creek
sand from a store
dirt from his own backyard
Carl pours one liter of water into each pot and measures how much water flows out of the bottom in one minute.

Which of the following variables should be kept constant during this experiment?
A.
the permeability of the soil in each pot
B.
the amount of water that goes into each pot
C.
the kind of soil in each pot
D.
the amount of water that comes out of each pot