Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.
Q: Problem: a diagonal of a polygon is an segment between two non - adjacent vertices of the polygon. how many diagonals does a regular 20 - sided polygon have ?
Options: ['a. 875', 'b. 170', 'c. 1425', 'd. 2025', 'e. 2500']
A:
b