6. There are ten equally spaced points on a circle. Then, among the convex quadrilaterals with four of these ten equally spaced points as vertices, the ratio of trapezoids is ( ).
(A) $\frac{8}{21}$
(B) $\frac{4}{21}$
(C) $\frac{1}{126}$
(D) $\frac{2}{7}$
6. D.

Choose any four points, there are $\mathrm{C}_{10}^{4}=210$ convex quadrilaterals, among which the two parallel sides of a trapezoid can be selected from five sets of 5 parallel chords parallel to the diameter, or from five sets of 4 parallel chords not parallel to the diameter, excluding rectangles, there are 60 trapezoids. Therefore, the proportion of trapezoids is $\frac{2}{7}$.