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: Military service is fulfilled by voluntary enlistment, generally for a period of ____ years.\Question: Choose the right answer from options given a) four b) one c) nine d) two
Example Output: c

Example Input: All spider mites have ____ legs and a lack of a separate head.\Question: Choose the right answer from options given a) eight b) four c) ten d) one
Example Output: a

Example Input: Fresh water is derived from ____ permanent rivers.\Question: Choose the right answer from options given a) six b) seven c) three d) four
Example Output:
c