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: Malignant tumors are more dangerous, and survival times can range from ____ months to several years.\Question: Choose the right answer from options given a) one b) no c) ten d) six
Example Output: d

Example Input: Aridity presents ____ main obstacles to solitary behavior.\Question: Choose the right answer from options given a) no b) three c) two d) five
Example Output: c

Example Input: Most vaccinations take effect in ____ weeks' time.\Question: Choose the right answer from options given a) two b) nine c) seven d) zero
Example Output:
a