19. 若效用函數 \(U(X, Y) = X^{\frac{1}{2}}Y^{\frac{3}{2}}\) ,則其 \(MRS_{XY} = ?\)(A) \(\frac{Y}{2X}\)(B) \(\frac{X}{2Y}\)(C) \(\frac{2Y}{X}\)(D) \(\frac{2Y}{3X}\) 。