Given the task definition and input, reply with output. 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.

Problem: a bag consists of 20 marbles, of which 5 are blue, 7 are red, and the remainder are white. if lisa is to select a marble from the bag at random, what is the probability that the marble will be red or white ?
Options: a. 3 / 4, b. 2 / 4, c. 1 / 4, d. 1 / 8, e. 1 / 16
a