Help understand how to solve question 4. ---**Question:** 4. Block A of mass m rests on the plank B of mass 3m which is free to slide on a frictionless horizontal surface. The coefficient of friction between the block and plank is 0.2. If a horizontal force of magnitude 2mg is applied to the plank B, the acceleration of A relative to the plank and relative to the ground respectively, are: **Options:** (a) 0, g/2 (b) 0, 2g/3 (c) 3g/5, g/5 (d) 2g/5, g/5 **Diagram Description:** * Type: Schematic diagram illustrating a physical setup. * Elements: * Block A: A rectangular block labeled "A" is shown resting on top of Plank B. It is labeled with mass "m". * Plank B: A larger rectangular plank labeled "B" is shown below Block A. It is labeled with mass "3m". * Surface: Plank B is shown resting on a horizontal surface represented by horizontal lines with small diagonal lines indicating it is a surface. This surface is described as frictionless in the text. * Applied Force: A horizontal arrow points to the right, applied to Plank B. It is labeled with "2mg". * Relative Positions: Block A is centered on top of Plank B. Plank B is resting on the horizontal surface. * Direction: The applied force is horizontal and acts towards the right.