Aresearcher 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