question: <image 1>which of the following is the area of the shaded region in the figure above?
  Options: 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: $\int_{0}^{a}\left[b-g(x)+f(x)\right]d x$