\[{{\left\{ {{\left( \frac{3}{4} \right)}^{-1}}-{{\left( \frac{1}{4} \right)}^{-1}} \right\}}^{-1}}=?\]

• A.  \[\frac{3}{8}\]                                   
• B.  \[\frac{-3}{8}\]                     
• C.  \[\frac{8}{3}\]                       
• D.  \[\frac{-8}{3}\]