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: Most children have a vocabulary of eight to ____ words, including names, by eighteen months of age.\Question: Choose the right answer from options given a) ten b) one c) seven d) eight
Example Output: a

Example Input: Females reach sexual maturity around ____ to five year-olds.\Question: Choose the right answer from options given a) four b) five c) one d) ten
Example Output: a

Example Input: Grasshoppers produce sound by ____ mechanisms called stridulation and crepitation.\Question: Choose the right answer from options given a) one b) six c) two d) eight
Example Output:
c