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: company x provides bottled water to its employees in 10 liter bottles, $ 2 each. at least how many 10 l bottles must company x buy monthly, so that new contract with $ 50 fixed monthly rate and $ 1 for 50 l bottle each paid off ? ( assume that no other costs apply )
Options: a. 28, b. 29, c. 30, d. 31, e. 32
a