As More Labor Is Hired, Moving Along the Production Function

As more labor is hired, moving along the production function, diminishing returns occur because

A) the wage rate paid is too low and so workers decrease their work effort.
B) workers are overworked and so their productivity decreases.
C) real GDP increases more rapidly the more workers are hired.
D) there are fixed quantities of other resources.
E) the real wage rate must increase in order to hire additional workers.