What is the system function of the system described by the difference equation y(n)=0.5y(n-1)+2x(n)?

• A. ( frac{2}{1+0.5z^{-1}} )
• B. ( frac{0.5}{1+2z^{-1}} )
• C. ( frac{0.5}{1-2z^{-1}} )
• D. ( frac{2}{1-0.5z^{-1}} )