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已知 $\sin \alpha-\cos \alpha=\frac{1}{5}, \alpha \in\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$ ，则 $\frac{\sin \alpha \cos \alpha}{\sin \alpha+\cos \alpha}=$
A. $-\frac{12}{5}$     B. $\frac{12}{5}$     C. $-\frac{12}{35}$     D. $\frac{12}{35}$         
不再提醒