6 From all the lines determined by any two of the 8 vertices of a cube, two lines are selected.
Then the probability that these two lines are skew lines is ( ).
(A) $\frac{29}{189}$
(B) $\frac{29}{63}$
(C) $\frac{34}{63}$
(D) $\frac{4}{7}$
6 From 8 vertices, any two points can determine $\mathrm{C}_{8}^{2}=28$ (lines). From 8 vertices, any 4 non-coplanar points can be chosen in $\mathrm{C}_{8}^{4}-12$ groups, and in each group of 4 non-coplanar points, there can be 3 pairs of skew lines. Therefore, the required probability is
$$
\frac{3\left(\mathrm{C}_{8}^{4}-12\right)}{\mathrm{C}_{28}^{2}}=\frac{29}{63} \text {. }
$$

So the answer is B.