6. Given that the price of a certain commodity increases by $x$ percent (1 percent being $10 \%$), the sales volume will decrease by $\frac{5}{6} x$ percent. To achieve the maximum turnover, the price should be increased by ( ) percent.
(A) 1
(B) 2
(C) 3
(D) 4
6.A.

Let the original selling price be $a$, and the original sales volume be $b$. Then the revenue is
$$
\begin{array}{l}
y=a(1+10\% x) \cdot b\left(1-\frac{5}{6} x \times 10\%\right) \\
=a b\left(1+\frac{x}{60}-\frac{x^{2}}{120}\right) .
\end{array}
$$

Since $a>0, b>0$, the revenue is maximized when $x=-\frac{\frac{1}{60}}{2 \times\left(-\frac{1}{120}\right)}=1$.