Figure 11-3 Fancy Footwear Manufactures Shoes

Figure 11-3 Fancy Footwear manufactures shoes. Figure 11-3 shows Fancy Footwear's marginal product of labor and average product of labor curves in the short run.
-Refer to Figure 11-3. For what quantity of labor does production start to display diminishing returns?

A) for more than 1 units of labor
B) for more than 4 units of labor
C) for more than 5 units of labor
D) for more than 8 units of labor