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]: Which of these is a structure that is present in all organisms?  
 (A) a tissue (B) a cell (C) a stomach (D) a brain
[EX A]: B

[EX Q]: A dish of sugar water was left on a window sill. One week later, there were only sugar crystals left in the dish. The water was gone. The best explanation for the appearance of sugar crystals is  
 (A) some of the water was absorbed by the dish. (B) the air condensed. (C) the light shining on the window sill produced more sugar. (D) the water evaporated.
[EX A]: D

[EX Q]: In which organism can cell walls be found?  
 (A) moth (B) grass (C) snake (D) hawk
[EX A]:
B