Chọn câu sai. Nếu $ frac{a}{b}= frac{c}{d}= frac{m}{n}$ thì:

A. $\frac{a}{b}=\frac{c}{d}=\frac{m}{n}=\frac{a+c+m}{b+d+n}$;          

B. $\frac{a}{b}=\frac{c}{d}=\frac{m}{n}=\frac{a+c-m}{b+n-d}$;            

C. $\frac{a}{b}=\frac{c}{d}=\frac{m}{n}=\frac{a+2c+m}{b+2d+n}$;                

D. $\frac{a}{b}=\frac{c}{d}=\frac{m}{n}=\frac{a-c-m}{b-d-n}$.