Sheila conducts an investigation to see how the amount of salt dissolved in water affects the density of the water. Which statement best describes the correct plan for her procedure?

The density and the amount of water are controls, and the amount of salt and the type of salt are variables.
The amount of water and the amount of salt are controls, and the type of salt and the density are variables.
The type of salt and the amount of water are controls, and the amount of salt and the density are variables.
The type of salt and the density of water are controls, and the amount of salt and the amount of water are variables.