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 weighs about ____ pounds per gallon.\Question: Choose the right answer from options given a) seven b) ten c) zero d) nine
Example Output: b

Example Input: Gestation lasts ____ months, with the birth of a single calf.\Question: Choose the right answer from options given a) three b) one c) six d) eight
Example Output: d

Example Input: Mammals Note the ____ vertebral regions - cervical, thoracic, lumbar, sacral and caudal.\Question: Choose the right answer from options given a) six b) nine c) five d) zero
Example Output:
c