Cho a = log35; b = log75. Khi đó khẳng định nào sau đây đúng

A. ${\log }_{15}\left(21\right)=\frac{\mathrm{a}+\mathrm{b}}{\mathrm{ab}+\mathrm{b}}$

B. ${\log }_{15}\left(21\right)=\frac{\mathrm{a}+\mathrm{b}}{\mathrm{a}+1}$

C. ${\log }_{15}\left(21\right)=\frac{\mathrm{a}-\mathrm{b}}{\mathrm{a}+1}$

D. ${\log }_{15}\left(21\right)=\frac{\mathrm{a}-\mathrm{b}}{\mathrm{ab}+\mathrm{b}}$