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').
--------
Question: Metals often crystallize in a ____ dimensional version of the hexagonal closest packed structure.\Question: Choose the right answer from options given a) one b) seven c) three d) eight

Answer: c

Question: Cows breed at ____ years, bulls at four.\Question: Choose the right answer from options given a) two b) three c) seven d) one

Answer: b

Question: Medical drugs can only kill one or ____ parasites each.\Question: Choose the right answer from options given a) two b) eight c) ten d) no

Answer:
a