A block of mass m is attached to the end of a weightless spring and placed on a frictionless horizontal surface. When the spring is compressed through a distance Δx from its unstretched length, it exerts a force F on the block. The block is released from rest from the position shown in the diagram. What is the speed of the block when the spring returns to its unstretched length? A. FΔx 2m B. FΔx m C. 2FΔx m D. 2 FΔx m