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').

Q: China has about ____ in four of all smokers in the world.\Question: Choose the right answer from options given a) one b) seven c) ten d) six

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

A: d
****
Q: Mammals are divided into ____ families, based on how their young develop.\Question: Choose the right answer from options given a) six b) ten c) one d) three

A:
d
****