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: Reverse osmosis uses ____ to ten gallons of water to produce one gallon of drinking water.\Question: Choose the right answer from options given a) two b) zero c) five d) three
Example Output: d

Example Input: Sexual maturity is reached at about ____ years of age.\Question: Choose the right answer from options given a) five b) ten c) no d) two
Example Output: d

Example Input: Litters consist of anywhere between ____ and six cubs.\Question: Choose the right answer from options given a) one b) ten c) no d) zero
Example Output:
a