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: Neutrinos are particles with almost ____ mass and no electric charge.\Question: Choose the right answer from options given a) no b) two c) zero d) five
Example Output: a

Example Input: Many soils have ____ major layers or horizons, top soil, subsoil and parent material.\Question: Choose the right answer from options given a) one b) two c) seven d) three
Example Output:
d