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: Litters contain from one to ____ kittens, and tend to be much larger when prey is abundant.\Question: Choose the right answer from options given a) three b) four c) two d) seven
Example Output: b

Example Input: Evolution occurs because of ____ forces.\Question: Choose the right answer from options given a) one b) three c) no d) six
Example Output:
b