What is the z-transform of the signal defined as x(n)=u(n)-u(n-N)?

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