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

[EX Q]: Ales actively ferment with yeast for two to ____ days.\Question: Choose the right answer from options given a) five b) nine c) three d) six
[EX A]: c

[EX Q]: Metamorphosis is simple with ____ stages of development.\Question: Choose the right answer from options given a) ten b) four c) six d) three
[EX A]:
d