You will be given a definition of a task first, then some input of the task.
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: 1.5, 2.5, 4.5, 7.5, 11.5, (.... )
Options: a. 16.5, b. 15.5, c. 14.5, d. 17.5, e. 17
Output:
a