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 man swims downstream 36 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water ?
Options: a. 7, b. 8, c. 9, d. 2, e. 4
Example Output: c

Example Input: Problem: a lady grows broccoli in her garden that is in the shape of a square. each broccoli takes 1 square foot of area in her garden. this year, she has increased her output by 79 broccoli when compared to last year. the shape of the area used for growing the broccoli has remained a square in both these years. how many broccoli did she produce this year ?
Options: ['a. 1600', 'b. 1500', 'c. 1650', 'd. 1700', 'e. can not be determined']
Example Output: a

Example Input: Problem: the distance from city a to city b is 120 miles. while driving from city a to city b, cara drives at a constant speed of 30 miles per hour. dan leaves city a 120 minutes after cara. what is the minimum constant speed in miles per hour that dan must exceed in order to arrive in city b before cara ?
Options: a. 42, b. 44, c. 60, d. 48, e. 50
Example Output:
c