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: Sexual reproduction involves ____ parents, one male and one female.\Question: Choose the right answer from options given a) zero b) two c) nine d) four
Example Output: b

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: Menstruation usually lasts between ____ and seven days.\Question: Choose the right answer from options given a) three b) seven c) eight d) no
Example Output:
a