6. The parallelogram $A B C D$ in the figure below is composed of 6 equilateral triangles. When it is folded along the dotted lines, a hexahedron in the shape of a rice dumpling as shown in the right figure can be obtained. In this hexahedron, the cosine value of the angle between $A B$ and $C D$ is ( )

A: 0
B: 1
C: $\frac{1}{2}$
D: $\frac{5}{6}$
Answer $\mathrm{C}$.
Analysis Take the middle dashed line $E F$, dividing the parallelogram into two parts, each consisting of three small equilateral triangles. The three small equilateral triangles on the left fold up (with $B$ and $F$ coinciding) to form exactly a bottomless regular triangular pyramid, with $A B, E F$ being its two base edges; similarly, the three small equilateral triangles on the right fold up (with $D$ and $E$ coinciding) to form the other half of the pyramid, with $C D, E F$ also being edge lines. Therefore, after folding, $A B, C D$ are two sides of the equilateral triangle, and the cosine of the angle between them is $\frac{1}{2}$.