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: how many kg of rice at rs. 6.60 per kg. be mixed with 56 kg of rice at rs. 9.60 per kg to get a mixture worth rs. 8.20 per kg
Options: a. 56 kg, b. 52 kg, c. 44 kg, d. 49 kg, e. none of these
d