AMC 8 2018 Problem 23

From a regular octagon, a triangle is formed by connecting three randomly chosen vertices of the octagon. What is the probability that at least one of the sides of the triangle is also a side of the octagon?

$\textbf{(A) } \frac{2}{7} \textbf{(B) } \frac{5}{42} \textbf{(C) } \frac{11}{14} \textbf{(D) } \frac{5}{7} \textbf{(E) } \frac{6}{7}$