\(B=\dfrac{9^3}{3^6.4}\)\(=\dfrac{\left(3^2\right)^3}{3^6.4}=\dfrac{3^6}{3^6.4}=\dfrac{1}{4}\)
\(C=\dfrac{4^5.9^4-2.6^9}{2^{10}.3^8+6^8.20}=\dfrac{\left(2^2\right)^5.\left(3^2\right)^4-2.\left(2.3\right)^9}{2^{10}.3^8+\left(2.3\right)^8.2^2.5}=\dfrac{2^{10}.3^8-2^{10}.3^9}{2^{10}.3^8+2^{10}.3^8.5}=\dfrac{\left(2^{10}.3^8\right)\left(1-3\right)}{\left(2^{10}.3^8\right)\left(1+5\right)}=\dfrac{-2}{6}=\dfrac{-1}{3}\)