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.

[EX Q]: 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.
[EX A]: C

[EX Q]: Which step will most likely ensure that the results of an investigation are valid?
(A) using a computer to graph results (B) verifying that results match the original hypothesis (C) comparing the results with those of other scientists (D) eliminating any results that appear to be abnormal
[EX A]: C

[EX Q]: A student drops a ball. Which force causes the ball to fall to the ground?
(A) electricity (B) friction (C) gravity (D) magnetism
[EX A]:
C