After Reading the Paragraph Below, Answer the Question That Follows

Multiple Choice

After reading the paragraph below, answer the question that follows.
You have purchased a brand new aquarium system. You bring it home, set it up, and fill it with clean tap water at the correct temperature. Excited to get this new hobby going, you ignore the advice of the man at the pet store who advised you to go slowly. You run out, purchase seven relatively large fish and put them in your aquarium. The next day, five of the seven fish are doing the backstroke. The day after, two fish have died and the remainder seem to be in poor health.
-What was the general problem that probably caused the fish deaths?

A) starvation due to absence of prey species
B) predation due to absence of prey species
C) nitrogen toxicity due to insufficient nitrogenous waste detoxifying bacterial populations
D) too high population density