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]: Electric circuits provide energy for lightbulbs. Which of these prevents the flow of electrons?  
 (A) a circuit that is closed (B) a series circuit (C) a circuit that is open (D) a parallel circuit
[A]: C


[Q]: Elephants on the African savanna eat shrubs and small trees. This behavior clears land and enables grass to grow. Grazers such as gazelles eat the grass. Lions prey on the gazelles. Which two organisms in this scenario are in the most cooperative relationship?   
 (A) the elephants and the shrubs (B) the shrubs and the lions (C) the elephants and the gazelles (D) the gazelles and the lions
[A]: C


[Q]: A star is a celestial object that produces its own heat and light. Which of these celestial objects is the closest star to Earth?  
 (A) Mars (B) Venus (C) the sun (D) the moon
[A]:
C