Two blocks with masses m1=0.25kg and m2=1.4kg approach each other along a horizontal, frictionless track. The initial velocities of the blocks are v1=1.20 m/s to the right and v2=0.34 m/s to the left. The two blocks then collide and stick together as shown in the figure. Which of the graphs could represent the force of block 1 on block 2 during the collision.