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: the total marks obtained by a student in mathematics and physics is 50 and his score in chemistry is 20 marks more than that in physics. find the average marks scored in mathamatics and chemistry together.
Options: a. 15, b. 25, c. 35, d. 45, e. 55

Ex Output:
c

Ex Input:
Problem: a student travels from his house to school at 12 km / hr and reaches school 2 hours late. the next day he travels 20 km / hr and reaches school 1 hour early. what is the distance between his house and the school ?
Options: a. 90, b. 100, c. 110, d. 120, e. 130

Ex Output:
a

Ex Input:
Problem: a bag consists of 30 marbles, of which 5 are blue, 9 are red, and the remainder are white. if lisa is to select a marble from the bag at random, what is the probability that the marble will be red or white ?
Options: a. 5 / 6, b. 2 / 4, c. 1 / 4, d. 1 / 8, e. 1 / 16

Ex Output:
a