19. 下列何選項為 $z = e^{\frac{x}{y}} + x^2 y^3$ 的全微分？
(A) $dz = (2xy^3 + \frac{1}{y}e^{\frac{x}{y}})dx + (3x^2y^2 - \frac{x}{y^2}e^{\frac{x}{y}})dy$
(B) $dz = (2xy^3 - \frac{1}{y}e^{\frac{x}{y}})dx + (3x^2y^2 + \frac{x}{y^2}e^{\frac{x}{y}})dy$
(C) $dz = (2xy^3 - \frac{x}{y^2}e^{\frac{x}{y}})dx + (3x^2y^2 + \frac{1}{y}e^{\frac{x}{y}})dy$
(D) $dz = (2xy^3 + \frac{x}{y}e^{\frac{x}{y}})dx + (3xy^2 + \frac{1}{y^2}e^{\frac{x}{y}})dy$

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