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 amount of water that comes out of each pot B. the amount of water that goes into each pot C. the permeability of the soil in each pot D. the kind of soil in each pot

In an experiment, there are factors known as constants that need to be the same all the time to test effectively the way the independent variable (factor changed) affects the dependent variable (factor studied). This, in the case of Carl means that to study the permeability of the soil (dependent variable) he manipulates the type of soil (independent variable), but other factors such as the type of pots, the amount of soil added and the water that goes into each pot needs to be same all time.

In the case of the amount of water, it is necessary the amount is the same because if Carl adds more water in one pot than in others it is likely more water filters not because of the soil being more permeable but because of the amount of water, and this can affect the results of the experiment. According to this, from the options given it is the amount of water that goes into each pot the variable that should be kept constant.