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: what is the rate percent when the simple interest on rs. 810 amount to rs. 155 in 4 years ?
Options: a. 4.78, b. 4.68, c. 4.58, d. 4.48, e. 4.38

Ex Output:
a

Ex Input:
Problem: a 24 ' ' x 24 ' ' square metal plate needs to be fixed by a carpenter on to a wooden board. the carpenter uses nails all along the edges of the square such that there are 25 nails on each side of the square. each nail is at the same distance from the neighboring nails. how many nails does the carpenter use ?
Options: ['a. 94', 'b. 95', 'c. 96', 'd. 97', 'e. 98']

Ex Output:
c

Ex Input:
Problem: a producer of tea blends two varieties of tea from two tea gardens one costing rs 18 per kg and another rs 20 per kg in the ratio 5: 3. if he sells the blended variety at rs 21 per kg, then his gain percent is
Options: a. 12 %, b. 13 %, c. 14 %, d. 15 %, e. 16 %

Ex Output:
a