The ruler is lying on two cylindrical crayons. Now imagine that you push one end of the ruler with your finger, causing it to move 4 cm forward. How far will crayons be moved forward?
Let’s imagine that crayons are not moving but could rotate on the spot. The ruler and the desk under crayons will start to move. We could move ruler 2 cm forward and desk 2 cm backward, instead to move the ruler 4 cm. Now, if desk moves relative to crayons the crayons could move relative to desk and if we consider desk immobile the crayons will move 2 cm forward relative to desk.
The answer is (C)