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 Q]: What characteristic of DNA results in cell differentiation in developing embryos?  
 (A) which genes are present (B) how many copies of each gene are present (C) which genes are active (D) what protein is produced by a gene
[EX A]: C

[EX Q]: What MAJOR advantage does nekton have over plankton in locating food?  
 (A) Nekton can actively swim. (B) Nekton can see in the dark. (C) Nekton do not eat much food. (D) Nekton can eat anything in the ocean.
[EX A]: A

[EX Q]: Trees most likely change the environment in which they are located by  
 (A) releasing nitrogen in the soil. (B) crowding out non-native species. (C) adding carbon dioxide to the atmosphere. (D) removing water from the soil and returning it to the atmosphere.
[EX A]:
D