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: Water is the only natural substance which exists in all ____ states of matter in nature.\Question: Choose the right answer from options given a) no b) eight c) six d) three
Example Output: d

Example Input: Ants have six legs, and each leg has ____ joints.\Question: Choose the right answer from options given a) nine b) zero c) two d) three
Example Output: d

Example Input: Scientists study the nervous system for ____ main reasons.\Question: Choose the right answer from options given a) ten b) one c) three d) two
Example Output:
d