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: Marriage is the legal union of ____ people.\Question: Choose the right answer from options given a) one b) no c) five d) two
Example Output: d

Example Input: Background radiation comes from ____ sources.\Question: Choose the right answer from options given a) zero b) three c) four d) no
Example Output: b

Example Input: Gastrulation begins ____ hours after fertilization.\Question: Choose the right answer from options given a) seven b) one c) no d) three
Example Output:
d