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.

[Q]: A potential negative impact of building a dam on a river is that the dam   
 (A) prevents sediment from flowing downstream. (B) increases the amount of water available to farms. (C) prevents seasonal downstream flooding. (D) increases the rate of water loss from a lake.
[A]: A


[Q]: Which equation is a balanced chemical equation?  
 (A) 2NaOH + CaBr_{2} -> Ca(OH)_{2} + NaBr (B) NO + Cl_{2} -> NOCl (C) CaCO_{3} -> CaO + 2CO_{2} (D) 2AlCl_{3} + 3Mg -> 3MgCl_{2} + 2Al
[A]: D


[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
[A]:
C