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.

Ex Input:
What occurs if the body's renal system fails to work?
(A) Gas exchange is impaired. (B) Blood oxygenation no longer occurs. (C) Metabolic waste builds up in the blood. (D) Nutrients are no longer transported to organs.

Ex Output:
C

Ex Input:
A student plans to design a new toothpaste tube that will give the same amount of toothpaste every time it is squeezed. Which should the student do first when designing the new tube?
(A) make a data table of amounts of toothpaste (B) gather building materials (C) create models of old toothpaste tubes (D) draw a few possible solutions

Ex Output:
D

Ex Input:
A group of students plans to build a model of a local pond habitat. Which model best represents an environment similar to a pond?
(A) a sealed plastic bottle containing insects and algae from a pond (B) a classroom aquarium containing plants and animals bought from a store (C) a classroom aquarium containing freshwater, non-native plants, and non-native animals (D) a small plastic outdoor pool containing freshwater, native plants, and native animals

Ex Output:
D