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]: 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.
[EX A]: A

[EX Q]: Scientists found a new species of a complex organism at the bottom of the ocean. The organism most likely  
 (A) is unicellular. (B) is a form of bacteria. (C) is made of multiple cells. (D) lacks an organized nucleus.
[EX A]: C

[EX Q]: What cell part is found in plant cells but not in animal cells?  
 (A) nucleus (B) cell wall (C) cytoplasm (D) cell membrane
[EX A]:
B