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: Real airplanes have round engines and ____ wings.\Question: Choose the right answer from options given a) four b) six c) three d) two
Example Output: d

Example Input: Natural indium contains ____ isotopes.\Question: Choose the right answer from options given a) one b) eight c) two d) seven
Example Output: c

Example Input: Insects live in warm climates, hot climates, even sub ____ climates.\Question: Choose the right answer from options given a) ten b) zero c) four d) eight
Example Output:
b