Tính B= 2^2/3. 3^2/8. 4^2/15. 6^2/35. 7^2/48. 8^2/63. 9^2.80 ta được

$B=\frac{2^2}{3}.\frac{3^2}{8}.\frac{4^2}{15}.\frac{5^2}{24}.\frac{6^2}{35}.\frac{7^2}{48}.\frac{8^2}{63}.\frac{9^2}{80}$

$=\frac{2.2}{1.3}.\frac{3.3}{2.4}.\frac{4.4}{3.5}.\frac{5.5}{4.6}.\frac{6.6}{5.7}.\frac{7.7}{6.8}.\frac{8.8}{7.9}.\frac{9.9}{8.10}$

$=\frac{\mathrm{2.3.4.5.6.7.8.9}}{\mathrm{1.2.3.4.5.6.7.8}}.\frac{\mathrm{2.3.4.5.6.7.8.9}}{\mathrm{3.4.5.6.7.8.9.10}}$

$=\frac{9}{1}.\frac{2}{10}=\frac{9.2}{1.10}=\frac{9}{5}$

Đáp án cần chọn là: A