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: Ground squirrels breed once a year, averaging ____ to eight per litter.\Question: Choose the right answer from options given a) seven b) one c) six d) ten
Example Output: a

Example Input: Scientists divide bats into ____ groups.\Question: Choose the right answer from options given a) five b) two c) one d) zero
Example Output: b

Example Input: Natural gas contains ____ atoms of hydrogen for every one atom of carbon.\Question: Choose the right answer from options given a) five b) ten c) one d) four
Example Output:
d