Question: which of the following is the area of the shaded region in the figure above? A) $\int_{0}^{a}\left[g(x)-f(x)\right]d x$  B) $\int_{0}^{a}\left[b+g(x)-f(x)\right]d x$  C) $\int_{0}^{a}\left[b-g(x)-f(x)\right]d x$  D) $\int_{0}^{a}\left[b-g(x)+f(x)\right]d x$. Please choose an answer from [A, B, C, D].
Answer: D