Which one of the following is not a formula of standard deviation $$\left( \sigma \right)?$$

A. $$\sigma = \frac{1}{N}\sqrt {\frac{{\sum {d^2}x}}{N} - {{\left( {\frac{{\sum dx}}{N}} \right)}^2}} $$

B. $$\sigma = \frac{1}{N}\sqrt {\sum {d^2}x - {{\left( {\sum dx} \right)}^2}} $$

C. $$\sigma = \sqrt {\frac{{\sum {d^2}x}}{N} - {{\left( {\overline x - A} \right)}^2}} $$

D. $$\sigma = \sqrt {\frac{{\sum {d^2}x - N{{\left( {\overline x - A} \right)}^2}}}{N}} $$

Answer: Option B