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').

[EX Q]: Ground squirrels breed once a year, averaging ____ to eight per litter.\Question: Choose the right answer from options given a) seven b) one c) six d) ten
[EX A]: a

[EX Q]: Chemical interactions are essentially electron interactions of ____ atoms.\Question: Choose the right answer from options given a) six b) no c) ten d) two
[EX A]: d

[EX Q]: Natural lithium contains ____ isotopes.\Question: Choose the right answer from options given a) zero b) eight c) two d) six
[EX A]:
c