Step-by-step explanation:
Without additional context or information, it's difficult to determine the exact reason the author included a description of Newton's first law of motion. However, based on the options provided, the most likely reason seems to be option D: to explain why the roller coaster car keeps moving up the hill despite gravity pulling it down. Newton's first law of motion, also known as the law of inertia, states that an object at rest will remain at rest unless acted upon by an external force. In the case of a roller coaster car moving up a hill, the car's initial speed and momentum help it to continue moving up the hill despite the force of gravity pulling it down. Understanding this concept can help explain the physics behind the roller coaster's motion.