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: Atoms are extremely small, on the order of a few ____ billionths of a meter.\Question: Choose the right answer from options given a) ten b) seven c) nine d) eight

A: a
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Q: Some European countries require ____ years of education.\Question: Choose the right answer from options given a) no b) nine c) six d) three

A: d
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Q: Asbestos is a mineral fibre which exists in ____ main forms.\Question: Choose the right answer from options given a) zero b) three c) no d) six

A:
b
****