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]: Most people require roughly seven to ____ hours of sleep a night to stay alert through the day.\Question: Choose the right answer from options given a) eight b) ten c) one d) five
[A]: a


[Q]: Echinoderms usually have ____ appendages, but there are some exceptions.\Question: Choose the right answer from options given a) ten b) five c) seven d) three
[A]: b


[Q]: Natural thallium is a mixture of ____ isotopes.\Question: Choose the right answer from options given a) ten b) five c) two d) six
[A]:
c