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.

Example Input: Problem: a rope of which a calf is tied is increased from 12 m to 23 m, how much additional grassy ground shall it graze ?
Options: a. 2989, b. 2789, c. 1210, d. 1288, e. 1211
Example Output: c

Example Input: Problem: there are 15 boys and 10 girls in a class. if three students are selected at random, in how many ways that 1 girl or 2 boys are selected ?
Options: a. 100, b. 105, c. 115, d. 120, e. 125
Example Output: c

Example Input: 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
Example Output:
a