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 Q]: A large number of species on Earth became extinct during a short time period about 65 million years ago. Based on this pattern of extinction, which is the most likely cause?  
 (A) gradual climate change (B) continued climate stability (C) sudden global climate change (D) gradual ecosystem succession
[EX A]: C

[EX Q]: Which statement best describes the development of a human embryo?  
 (A) A single cell becomes many cells. (B) Each cell has a different function. (C) Many cells combine to form one cell. (D) All the cells have the same function.
[EX A]: A

[EX Q]: The crews of two boats at sea can communicate with each other by shouting. Why is it impossible for the crews of two spaceships a similar distance apart in space to do this?  
 (A) The sound is reflected more in space. (B) The pressure is too high inside the spaceships. (C) The spaceships are traveling faster than sound. (D) There is no air in space for the sound to travel through.
[EX A]:
D