You are given a science question (hard-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Q: Around areas of increased fertilizer runoff, all of the heterotrophs within a lake food chain may show an increase in numbers. After the initial increase, the populations begin to decline. Which event most likely caused the populations to decline?
(A) More predators were introduced into the lake. (B) Water was depleted of nutrients such as nitrogen. (C) Breeding season for the organisms came to an end. (D) Decomposing algae lowered the amount of dissolved oxygen.
A:
D