6. The same product at the same price underwent two price adjustments in three malls. Mall A: The first price increase percentage is $a$, and the second price increase percentage is $b$; Mall B: The price increase percentage for both times is $\frac{a+b}{2}(a>0, b>0)$; Mall C: The first price increase percentage is $b$, and the second price increase percentage is $a$. Then, the mall with the highest price increase is ( ).
(A) A
(B) B
(C) C
(D) Cannot be determined
6.B.

After two price increases, the price at Store A is
$$
\begin{array}{l}
(1+a)(1+b) \\
=1+a+b+a b ;
\end{array}
$$

After two price increases, the price at Store B is
$$
\left(1+\frac{a+b}{2}\right)\left(1+\frac{a+b}{2}\right)=1+(a+b)+\left(\frac{a+b}{2}\right)^{2} \text {; }
$$

After two price increases, the price at Store C is
$$
(1+b)(1+a)=1+a+b+a b \text {. }
$$

Since $\left(\frac{a+b}{2}\right)^{2}-a b>0$, it follows that $\left(\frac{a+b}{2}\right)^{2}>a b$.
Therefore, after two price increases, the price at Store B is the highest.