A：$n^2-\frac{1}{2^n}+1$

B：$n^2-\frac{1}{2^{n+1}}+\frac{1}{2}$

C：$n^2-n-\frac{1}{2^n}+1$

D：$n^2-n-\frac{1}{2^{n+1}}+\frac{1}{2}$