The Following Table Shows the Marginal Productivity of Different Units

The following table shows the marginal productivity of different units of labor for producing a product. Table 28.4
In the table,
MR: Marginal Revenue
MPP: Marginal Physical Product
According to Table 28.4, how many laborers should be hired if the wage rate is $18? (Marginal revenue product = MPP ´ MR)

A) 2
B) 3
C) 4
D) 5
E) 6