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: Circles mark the approximate locations of ____ recently discovered solar systems.\Question: Choose the right answer from options given a) four b) no c) zero d) nine
Example Output: a

Example Input: Human females have ____ X chromosomes.\Question: Choose the right answer from options given a) two b) six c) one d) eight
Example Output: a

Example Input: Selenium exists in several allotropic forms, although ____ are generally recognized.\Question: Choose the right answer from options given a) eight b) two c) no d) three
Example Output:
d