Use the Following Figure to Answer the Question

Multiple Choice

Use the following figure to answer the question.

After looking at the results in the figure, what can be concluded about productivity in the experimental ecosystem?

A) Productivity is higher at depth than at the surface, but only initially.
B) Productivity is higher at the surface than at depth; however, an additional experiment could further test the effect of depth by introducing FeSO₄ at 30 m deep and measuring the chlorophyll a response.
C) Productivity is higher at the surface than at depth, and would be predicted to increase sharply after day 12.
D) Productivity at a depth of 30 m is low only because of a lack of sunlight, not a lack of iron sulfate.