D. Because science only uses facts that are proven 100% true through the use of variables.
Explanation:
All the knowledge we read is science books are not merely some observed facts but the result of a systematic scientific methodology through which facts are tested. Science only gives us that knowledge as facts which is proven 100% true through the use of variables.For example: if a scientist has studied the effect of salt stress on the growth of plants and he got the results in form of graphs which show that growth of salt treated plants in very less than normal plants. Then he needs to draw conclusion after consulting relevant previous experiments and say that salt stress hinders growth and why? with reasonable logic.
Variables are the things hat he changes like environmental conditions in previous example to see that in changed conditions plant is showing changed behavior. This way a scientist strengthens his results. Therefore, we can say that whatever knowledge we get through science is proven 100% true through the use of variables.
