20. An object will stay at rest, or keep moving at the same speed & in a straight line, unless it is acted upon by an external unbalanced force. This is known as A Friction B Force C Gravity D Inertia

<p> This question covers the basic principles of motion in physics. It describes Newton's first law, also known as the law of inertia. This law states that "An object will stay at rest or keep moving at the same speed in a straight line, unless acted upon by an external unbalanced force". The options given are various factors that could potentially influence motion, but only one accurately encapsulates the described principle.</p>