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: roses can be purchased individually for $ 6.30, one dozen for $ 36, or two dozen for $ 50. what is the greatest number of roses that can be purchased for $ 680 ?
Options: a. 156, b. 162, c. 316, d. 324, e. 325
c