[NO LINKS] John and Emily were experimenting with the effects of road salt on plant growth. To do this, they watered plants daily with the same amount of salt water. They could best improve their experiment by Question 16 options:

increasing the amount of salt added to the water each day

cooling the salt water before watering the plants

adding food color to see how far the salt penetrates the plants

watering half the plants with pure water and half with salt water