TASK DEFINITION: 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.
PROBLEM: Which two protein molecules are essential for muscle contraction and relaxation?  
 (A) actin and myosin (B) collagen and tubulin (C) hemoglobin and myoglobin (D) amylase and polymerase

SOLUTION: A

PROBLEM: Which occurs as a result of Earth's tilt on its rotating axis?  
 (A) movement of the tides (B) prevalent or trade winds (C) seasonal changes in the climate (D) light and dark changes of day and night

SOLUTION: C

PROBLEM: Which activity illustrates the main function of the motor neuron complex?  
 (A) transferring impulses to the appropriate muscle (B) receiving sensory impulses from the environment (C) delivering regular impulses to the autonomic system (D) processing impulses within the cerebral cortex

SOLUTION:
A