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: Voles are prolific breeders and can produce four to ____ litters a year.\Question: Choose the right answer from options given a) four b) two c) six d) three
Example Output: c

Example Input: Sexual reproduction is varied in ____ ways.\Question: Choose the right answer from options given a) seven b) two c) five d) three
Example Output: d

Example Input: Dolphins use frequencies ____ thousand times as great to locate prey.\Question: Choose the right answer from options given a) seven b) one c) ten d) four
Example Output:
c