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: Nightmares happen frequently in childhood, especially between the ages of ____ and six.\Question: Choose the right answer from options given a) three b) eight c) no d) ten
Example Output: a

Example Input: Songbirds have ____ sets of vocal chords.\Question: Choose the right answer from options given a) six b) two c) eight d) ten
Example Output: b

Example Input: Most insects have one or ____ pairs of wings.\Question: Choose the right answer from options given a) three b) zero c) ten d) two
Example Output:
d