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]: The nucleus of the cell controls cell activity. Which body system performs a similar function in humans?  
 (A) circulatory system (B) central nervous system (C) skeletal system (D) integumentary system
[EX A]: B

[EX Q]: Which pair of organisms benefits each other the most?  
 (A) birds building a nest in a bush (B) worms eating the roots of a plant (C) squirrels burying seeds from a tree (D) deer scraping their antlers on a tree
[EX A]: C

[EX Q]: The gravitational force of the Sun affects the planets in our solar system. Which of these is influenced the most by this force?  
 (A) axial tilt (B) orbital path (C) the masses of the planets (D) the number of moons per planet
[EX A]:
B