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已知 $\frac{\sin \alpha+\cos \alpha}{\cos \alpha}=\sqrt{2}$ ,则 $\tan \left(\alpha-\frac{\pi}{4}\right)=$
A. $\sqrt{2}-1$     B. $1-\sqrt{2}$     C. $\frac{\sqrt{2}}{2}-1$     D. $1-\frac{\sqrt{2}}{2}$         
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