Definition: 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.
Input: Wolves, which are top predators, were eliminated from Yellowstone National Park in the 1930s. In 1995, wolves were reintroduced into Yellowstone. During the period in which wolves were absent from Yellowstone, which most likely occurred?
(A) an increase in competition for food resources among small prey (B) a greater opportunity for primary producers to flourish (C) an increase in the population of tertiary consumers (D) a greater balance of predator-prey relationships
Output:
A