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: Polyester has ____ favorable properties.\Question: Choose the right answer from options given a) four b) two c) seven d) six
Example Output: b

Example Input: Most gerbils live three to ____ years and have few health problems.\Question: Choose the right answer from options given a) three b) four c) seven d) zero
Example Output: b

Example Input: Echocardiography demonstrates in clear, graphic fashion the ____ cardiac chambers and four valves.\Question: Choose the right answer from options given a) one b) two c) six d) four
Example Output:
d