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: Spores have ____ combinations of sexes.\Question: Choose the right answer from options given a) four b) five c) seven d) three
Example Output: a

Example Input: Humans normally produce only ____ sets of teeth.\Question: Choose the right answer from options given a) nine b) eight c) two d) three
Example Output: c

Example Input: Nitrates can be especially toxic to children under ____ months of age.\Question: Choose the right answer from options given a) nine b) six c) three d) one
Example Output:
b