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 couple who own an appliance store discover that if they advertise a sales discount of 10 % on every item in the store, at the end of one month the number of total items sold increases 15 %. their gross income from sales for one month increases by what percent ?
Options: a. 1.5 %, b. 2.5 %, c. 3.5 %, d. 4.5 %, e. 5.5 %

Ex Output:
c

Ex Input:
Problem: if the simple interest on a certain sum of money for 6 years is one fifth of the sum, then the rate of interest per annum is
Options: a. 3.3 %, b. 7.2 %, c. 6.1 %, d. 5.0 %, e. 3.1 %

Ex Output:
a

Ex Input:
Problem: a one - foot stick is marked in 1 / 4 and 1 / 5 portion. how many total markings will there be, including the end points ?
Options: a. 18, b. 16, c. 13, d. 9, e. 11

Ex Output:
d