Please Use the Following Information to Answer the Questions Below

Multiple Choice

Please use the following information to answer the questions below.
A scientist conducts an experiment to quantify the effect of mycorrhizae and soil nitrogen levels on plant growth in a grassland.The experiment has four treatments: (1) mycorrhizae present and ambient levels of nitrogen, (2) mycorrhizae present and nitrogen added, (3) mycorrhizae absent and ambient levels of nitrogen,and (3) mycorrhizae absent and nitrogen added.After four weeks,the scientist removes the aboveground plant material for weighing.
Figure 42.4
-Figure 42.4 shows the results of the experiment.What is (are) the independent variable(s) ?

A) nitrogen treatment only
B) biomass of aboveground plant only
C) presence and absence of mycorrhizae
D) nitrogen treatment and whether mycorrhizae are present