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: Redwoods reproduce ____ ways.\Question: Choose the right answer from options given a) zero b) two c) six d) nine
Example Output: b

Example Input: Natural indium contains ____ isotopes.\Question: Choose the right answer from options given a) one b) eight c) two d) seven
Example Output: c

Example Input: Many swimmers train for more than ____ years during their careers.\Question: Choose the right answer from options given a) ten b) two c) nine d) seven
Example Output:
a