What is the magnitude of the frequency response of the system described by the difference equation y(n)=ay(n-1)+bx(n), 0<a<1?

• A. ( frac{|b|}{ sqrt{1+2acos +a^2}} )
• B. ( frac{|b|}{1-2acos +a^2} )
• C. ( frac{|b|}{1+2acos +a^2} )
• D. ( frac{|b|}{ sqrt{1-2acos +a^2}} )