Reflection coefficient of plane waves with normal incidence in terms of impedances of medium is ____

• A. ( _r= frac{ _2- _1}{ _2+ _1} )
• B. ( _r= frac{2 _2}{ _2+ _1} )
• C. ( _r= frac{2 _1}{ _2+ _1} )
• D. ( _r= frac{ _2+ _1}{ _2- _1} )