Either A or D. If I were answering I'd go with my git answer and say A
Answer:
Friction
Explanation:
Friction is a force that slows down moving objects. If you roll a ball across a shaggy rug, you can see that there are lumps and bumps in the rug that make the ball slow down. The rubbing, or friction, between the ball and the rug is what makes the ball stop rolling. External Force is required.
Answer:
The center of mass of the Earth–Moon system is 4.613 × 10⁶ m from center of the Earth.
Explanation:
Let the reference point be the center of the Earth

Where;
Xcm is the distance from center of the Earth =?
Me is the mass of the Earth = 6 × 10²⁴ kg
Xe is the center mass of the Earth = 0
Mm is the mass of the moon = 7 × 10²² kg
Xm is the center mass of the moon = 4 × 10⁸ m

Therefore, the center of mass of the Earth–Moon system is 4.613 × 10⁶ m from center of the Earth.