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: Electrons have almost ____ mass and are negatively charged.\Question: Choose the right answer from options given a) three b) seven c) zero d) no
Example Output: d

Example Input: Mammals have only ____ sets of teeth.\Question: Choose the right answer from options given a) six b) five c) seven d) two
Example Output: d

Example Input: Rats have estrous cycles throughout the year, each lasting just ____ days.\Question: Choose the right answer from options given a) eight b) six c) nine d) five
Example Output:
d