若 $\cos \left(\frac{\pi }{4}-\alpha \right)=\frac{3}{5}$ ，则 $\mathit{sin}2\alpha =$ ( )

A．

$\frac{7}{25}$

B．

$\frac{1}{5}$

C．

$-\frac{1}{5}$

D．

$-\frac{7}{25}$