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.

[Q]: What are produced in both the male and female reproductive systems in humans?  
 (A) sperm (B) nutrients (C) fertilized eggs (D) sex cells
[A]: D


[Q]: Which of these is the center of our solar system?  
 (A) Mars (B) The moon (C) Jupiter (D) The sun
[A]: D


[Q]: What is one way to help slow down or stop the greenhouse effect?  
 (A) find new coal resources in the earth (B) clean up the litter in neighborhoods (C) clean up the water pollution in local lakes (D) walk or ride a bike instead of driving a car
[A]:
D