In this task, you need to answer the given multiple-choice question on the general math. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Ex Input:
Problem: if ( a - b - c + d = 13 ) and ( a + b - c - d = 9 ) , what is the value of ( b - d ) ^ 2 ? 
Options: a ) 4 . , b ) 8 . , c ) 12 . , d ) 16 . , e ) 64 .

Ex Output:
a


Ex Input:
Problem: 49 ã — 49 ã — 49 ã — 49 x 49 = 7 ? 
Options: a ) 4 , b ) 7 , c ) 8 , d ) 10 , e ) none of these

Ex Output:
d


Ex Input:
Problem: the average mark of the students of a class in a particular exam is 80 . if 5 students whose average mark in that exam is 20 are excluded , the average mark of the remaining will be 92 . find the number of students who wrote the exam . 
Options: a ) 15 , b ) 25 , c ) 30 , d ) 45 , e ) 55

Ex Output:

c