6. Let $a, b, c \in \mathbf{R}_{+}$, and $a b + b c + c a = 108$. Then the minimum value of $\frac{a b}{c} + \frac{b c}{a} + \frac{c a}{b}$ is ( ).
(A) 6
(B) 12
(C) 18
(D) 36
6.C.
$$
\begin{array}{l}
S=\frac{(a b)^{2}+(b c)^{2}+(c a)^{2}}{a b c} \\
\geqslant \frac{a b \cdot b c+b c \cdot c a+c a \cdot a b}{a b c} \\
=a+b+c=\sqrt{(a+b+c)^{2}} \\
\geqslant \sqrt{3(a b+b c+c a)}=\sqrt{324}=18 .
\end{array}
$$