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.

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

In the late 1700s, Antoine Lavoisier performed a series of experiments to find out what happened when a substance burned. In each experiment, he observed that the weight of a container and its contents was the same at the end of the experiment as it had been in the beginning. These observations led him to propose the law of  
 (A) combustion. (B) fixed proportions. (C) conservation of mass. (D) conservation of energy.
C

In a closed system, what happens to the total energy of the system as energy conversions take place?  
 (A) It increases. (B) It decreases. (C) It remains constant. (D) It is transferred out of the system.

C