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: Brachiopods are superficially similar to bivalves, both having ____ shells.\Question: Choose the right answer from options given a) two b) eight c) seven d) one
Example Output: a

Example Input: Mothers usually give birth to one, two, or ____ babies at a time.\Question: Choose the right answer from options given a) three b) zero c) five d) ten
Example Output: a

Example Input: Birds have ____ types of nephrons.\Question: Choose the right answer from options given a) seven b) six c) two d) four
Example Output:
c