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').

Example Input: Antibodies contain ____ pairs of protein chains.\Question: Choose the right answer from options given a) no b) two c) nine d) zero
Example Output: b

Example Input: Dunes are of ____ types - linear and parabolic.\Question: Choose the right answer from options given a) three b) four c) ten d) two
Example Output: d

Example Input: National immunisation vaccinates all children under ____ against polio.\Question: Choose the right answer from options given a) three b) five c) one d) four
Example Output:
d