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: Pandas reach adulthood at about ____ years of age.\Question: Choose the right answer from options given a) eight b) seven c) five d) six
Example Output: c

Example Input: Chlorine has ____ naturally occurring isotopes.\Question: Choose the right answer from options given a) nine b) eight c) three d) two
Example Output: d

Example Input: Tetrapods are animals that have ____ legs or leg-like appendages .\Question: Choose the right answer from options given a) six b) four c) ten d) seven
Example Output:
b