Chọn câu đúng? (x^2 - 3xy)/(21y^2 - 7xy) = x/7y

A. $\frac{{\text{x}}^{\text{2}}-\text{3xy}}{{\text{21y}}^{\text{2}}-\text{7xy}}=\frac{\text{x}}{\text{7y}}$

B. $\frac{\text{2x}+\text{4}}{{\text{x}}^{\text{2}}-\text{x}-\text{6}}=\frac{\text{4}}{\text{x}-\text{3}}$

C. $\frac{\text{2x}-\text{6y}}{{\text{x}}^{\text{2}}-{\text{9y}}^{\text{2}}}=\frac{\text{2}}{\text{x}+\text{9y}}$

D. $\frac{{\text{x}}^{\text{2}}-\text{2x}}{{\text{x}}^{\text{3}}-\text{4x}}=\frac{\text{1}}{\text{x}+\text{2}}$