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.

Ex Input:
Problem: a boat having a length 8 m and breadth 2 m is floating on a lake. the boat sinks by 1 cm when a man gets on it. the mass of the man is :
Options: a. 12 kg, b. 60 kg, c. 72 kg, d. 160 kg, e. none of these

Ex Output:
d

Ex Input:
Problem: 1 men takes 37 $ to eat a pizza in restaurant such that it have discount multiple of 4 if u eat more pizza. discount increase by multiple of 4 every pizza u eat. for 1 pizza 0 % off. for 2 pizza % off for 3 pizza 8 % off. pizza without discount is 10 $. how much that man eats the pizza.
Options: a. 2, b. 3, c. 4, d. 5, e. 6

Ex Output:
b

Ex Input:
Problem: solve the equation 2 x = 2 b 2 ax, where a and b are real parameters. find for which values of a the equation has for solution a natural number, if b = 7
Options: a. 0, b. 1, c. 2, d. 3, e. 4

Ex Output:
b