What was the significance of Newton's third law of motion?

What was the significance of Newton's third law of motion?
A、Every action has an equal and opposite reaction.
B、An object at rest will stay at rest, and an object in motion will stay in motion.
C、Gravity is not affected by mass.
D、The force of gravity is directly proportional to the distance between two objects.
【正确答案】：A
【题目解析】：Every action has an equal and opposite reaction.