A Key Characteristic of the Production Function in the Endogenous

A key characteristic of the production function in the endogenous growth model presented in the text is that

A) there are constant returns to scale in human capital.
B) there are decreasing returns to scale in human capital.
C) at low levels of human capital, there are increasing returns to scale in human capital, while at high levels of human capital, there are decreasing returns to scale in human capital.
D) there are increasing returns to scale in human capital.
E) there are substantial diminishing marginal returns to human capital.