A：$\sum _{i=1}^n{\left(\frac{i-1}{n}\right)}^2·\frac{1}{n}$

B：$\underset{n\to \infty }{\lim }\sum _{i=1}^n{\left(\frac{i-1}{n}\right)}^2·\frac{1}{n}$

C：$\sum _{i=1}^n{\left(\frac{2i}{n}\right)}^2·\frac{2}{n}$

D：$\underset{n\to \infty }{\lim }\sum _{i=1}^n{\left(\frac{2i}{n}\right)}^2·\frac{2}{n}$