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: Respiratory therapists complete two to ____ years of education.\Question: Choose the right answer from options given a) no b) one c) four d) eight
Example Output: c

Example Input: Government buildings fly flags of the ____ nations.\Question: Choose the right answer from options given a) ten b) no c) seven d) two
Example Output: d

Example Input: Ionic mercury can exist in ____ forms.\Question: Choose the right answer from options given a) two b) zero c) ten d) five
Example Output:
a