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 and b are two partially filled buckets of water. if 5 liters are transferred from a to b, then a would contain one - third of the amount of water in b. alternatively, if 4 liters are transferred from b to a, b would contain one - half of the amount of water in a. bucket a contains how many liters of water ?
Options: a. 11, b. 52 / 5, c. 17, d. 21, e. 23
A:
b