Cho dãy số 

Câu hỏi: Cho dãy số $\left(u_n\right)$ với $u_n=3^{\frac{n}{2}+1}$ . Tính tổng $S=u_2+u_4+u_6+...+u_{20}$

A. $S=\frac{9}{2}\left(3^{20}+1\right)$

B. $S=\frac{9}{2}\left(3^{20}-1\right)$

C. $S=\frac{9}{2}\left(3^{10}-1\right)$

D. $S=\frac{7}{2}\left(3^{10}-1\right)$