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: Mechanical digestion occurs as ____ strong layers of muscle contract to produce a churning motion.\Question: Choose the right answer from options given a) four b) no c) three d) six
Example Output: c

Example Input: Some taxonomists classify only ____ species though.\Question: Choose the right answer from options given a) one b) six c) two d) three
Example Output: c

Example Input: Autism is a developmental disability identified during the first ____ years of life.\Question: Choose the right answer from options given a) no b) six c) four d) three
Example Output:
d