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: Female elephants have babies about ____ years apart, and they have only one each time.\Question: Choose the right answer from options given a) eight b) one c) zero d) three
Example Output: d

Example Input: Some sources claim there are more like ____ million vertebrate animals in zoos.\Question: Choose the right answer from options given a) six b) five c) one d) three
Example Output: b

Example Input: Wolves usually live in packs of ____ to twelve.\Question: Choose the right answer from options given a) three b) two c) five d) nine
Example Output:
b