Transmission coefficient of a plane wave with normal incidence is _________

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