Can you explain why answer wouldn't be 3?

Labor Output Fixed Cost Variable Cost Total Cost (Number of workers) (Units) (Dollars) (Dollars) (Dollars) 0 0 50 0 50 90 50 20 70 170 50 40 90 230 50 60 110 240 50 80 130 Refer to Table 13-3. At which number of workers does diminishing marginal product begin? O O w O