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: Batteries consist of ____ electrodes immersed in an electrolyte.\Question: Choose the right answer from options given a) two b) eight c) three d) seven
Example Output: a

Example Input: Most marine sediments are of ____ basic types.\Question: Choose the right answer from options given a) three b) ten c) two d) nine
Example Output: c

Example Input: Casein proteins have ____ major types, and all three appear in every mammal species on Earth.\Question: Choose the right answer from options given a) one b) no c) three d) four
Example Output:
c