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