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: Pupae usually transform into adults ____ to three days later.\Question: Choose the right answer from options given a) one b) two c) no d) eight
Example Output: b

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: Iron occurs in ____ forms in food.\Question: Choose the right answer from options given a) zero b) two c) no d) four
Example Output:
b