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 worker earns $ 20 on the first day and spends $ 15 on the second day. the worker earns $ 20 on the third day and spends $ 15 on the fourth day. if this pattern continues, on which day will the worker first reach a net total of $ 80 ?
Options: a. 24, b. 25, c. 27, d. 30, e. 32
Example Output: b

Example Input: Problem: in what time will a train 120 m long cross an electric pole, it its speed be 121 km / hr ?
Options: a. 2.5, b. 2.7, c. 2.9, d. 3.6, e. 2.1
Example Output: d

Example Input: Problem: if a is a positive integer, and if the units digit of a ^ 2 is 9 and the units digit of ( a + 1 ) ^ 2 is 6, what is the units digit of ( a + 2 ) ^ 2 ?
Options: a. 1, b. 3, c. 5, d. 6, e. 8
Example Output:
c