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.

Example Input: A bright object with a long tail of glowing gases is in orbit around the Sun. This object is most likely  
 (A) a planet (B) a star (C) an asteroid (D) a comet
Example Output: D

Example Input: Use of nuclear fuel would most likely reduce human contributions to which environmental problem?  
 (A) ozone thinning (B) landfill creation (C) shoreline erosion (D) greenhouse heating
Example Output: D

Example Input: In areas where limestone is present underneath the soil, continued rains can alter the limestone formation. The limestone can change to a highly porous formation that can lead to the formation of caves and sinkholes. Which process is directly responsible for changing the limestone?  
 (A) compaction (B) erosion (C) faulting (D) weathering
Example Output:
D