Students are experimenting with a substance that they think can be added to soil to increase the number of flowers a certain plant will make. They set up 20 pots of soil, each planted with the same type of seed. They divide the pots into two groups and add 15 drops of the substance to pots in one group and only 10 drops to pots in the other group. After the plants grow for four weeks, the students count how many flowers each plant has made. What does this experimental investigation lack?

a control group
an independent variable
a dependent variable
a procedure