You are given a science question (easy-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:
A student examines a new watch design. The watch has a small solar cell on its face. The main function of the solar cell is to  
 (A) reduce the cost of the watch. (B) provide an energy source for the watch. (C) make the watch more attractive. (D) stop the watch from speeding up.

Ex Output:
B


Ex Input:
One type of animal hatches from an egg, breathes through gills when it is young, and mainly lives on land as an adult. Into which group is this animal classified?  
 (A) amphibians (B) birds (C) mammals (D) reptiles

Ex Output:
A


Ex Input:
What keeps stars such as the Sun from collapsing from their own self-gravity?  
 (A) the centrifugal force created by rapid rotation (B) the electrical repulsion of nuclei in the plasma (C) the gravitational pull created by orbiting planets (D) the outward pressure created by nuclear fusion

Ex Output:

D