Cho \({\left( {x + \frac{1}{2}} \right)^{40}} = \sum\limits_{k = 0}^{40} {{a_k}{x^k}} ,\) với \({a_k} \in R\).

A. \({a_{25}} = {2^{25}}C_{40}^{25}\)

B. \({a_{25}} = \frac{1}{{{2^{25}}}}C_{40}^{25}\)

C. \({a_{25}} = \frac{1}{{{2^{15}}}}C_{40}^{25}\)

D. \({a_{25}} = C_{40}^{25}\)