A researcher is interested in the effects of nitrate and phosphate on plant growth and sets up an experiment in which groups of five plants are given 1, 2, and 3 grams of nitrate and 1, 2, and 3 grams of phosphate in all combinations over a period of one month. The researcher measures plant height and weight at the end of the experiment. What is missing in this experimental design?
Question 9 options:
a control
an independent variable
a dependent variable
a constant