Given a sentence with a missing word, pick the answer option that best fills out the missing word in the sentence. Indicate each answer with its index ('a', 'b', 'c', 'd').

Ex Input:
Background radiation comes from ____ sources.\Question: Choose the right answer from options given a) zero b) three c) four d) no

Ex Output:
b


Ex Input:
Younger children can produce as much as ____ times more saliva than adults.\Question: Choose the right answer from options given a) one b) ten c) five d) eight

Ex Output:
d


Ex Input:
Volume units are cubic units, since they are the result of multiplying ____ units of length.\Question: Choose the right answer from options given a) one b) three c) four d) six

Ex Output:

b