The area of the shaded region bounded by the curves $y^2 = x$, $x = 4$ and the $x$-axis is given by (A) $\int_{0}^{4} x \, dx$ (B) $\int_{0}^{2} y^2 \, dy$ (C) $2 \int_{0}^{4} \sqrt{x} \, dx$ (D) $\int_{0}^{4} \sqrt{x} \, dx$