In David Tilman's Experiments with the Diatoms Cyclotella and Asterionella

Multiple Choice

In David Tilman's experiments with the diatoms Cyclotella and Asterionella, when the diatoms were grown at silicon/phosphorus ratios between 6 and 90, why did the two species coexist?

A) Both species were phosphorus limited.
B) Both species were silicon limited.
C) Neither species was limited by phosphorus or silicon.
D) One species was limited by phosphorus, and the other was limited by silicon.
E) Tilman continually supplemented the cultures with individuals of the two species.