To test the lever-arm model of myosin movement, an investigator utilizes recombinant DNA technology to attach myosin head to various length of neck domains. In the schematics shown below (1-4), the y-axis represents the velocity of myosin in μm/sec on the actin filament, and the x-axis shows the recombinant myosins (a-d; shown on the left) that were utilized to calculate the velocity. Considering that all the appropriate conditions were applied to estimate the velocity of recombinant myosin, choose the graph that correctly represents the velocity of recombinant myosin.