Cho \[A = \frac{{1.3.5.7...39}}{{21.22.23...40}}\] và \[A = \frac{{1.3.5...\left( {2n - 1} \right)}}{{\left( {n + 1} \right)\left( {n + 2} \right)\left( {n + 3} \right)...2n}}\left( {n \in N*} \right)\]
Chọn câu đúng.
A. \[A = \frac{1}{{{2^{20}}}};B = \frac{1}{{{2^n}}}\]
B. \[A = \frac{1}{{{2^{25}}}};B = \frac{1}{{{2^{n + 1}}}}\]
C. \[A = \frac{1}{{{2^{20}}}};B = \frac{1}{{{2^{2n}}}}\]
D. \[A = \frac{1}{{{2^{21}}}};B = \frac{1}{{{2^{n + 1}}}}\]