If $$\frac{x}{y}=\frac{c}{d}$$ and $$\frac{d}{c}=\frac{b}{a}$$, which of the following must be true?
I. $$\frac{y}{x} = \frac{b}{a}$$
II. $$\frac{x}{a} = \frac{y}{b}$$
III. $$\frac{y}{a} = \frac{x}{b}$$
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