Suy luận nào sau đây đúng? a>b và x>d => ac>bd

A. $\left\{\begin{array}{c}a>b\\ c>d\end{array}\right.\Rightarrow ac>bd$

B. $\left\{\begin{array}{c}a>b\\ c>d\end{array}\right.\Rightarrow \frac{a}{c}>\frac{b}{d}$ 

C. $\left\{\begin{array}{c}a>b\\ c>d\end{array}\right.\Rightarrow a-c>b-d$

D. $\left\{\begin{array}{c}a>b>0\\ c>d>0\end{array}\right.\Rightarrow ac>bd$