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]: Perennial grass takes three to ____ years before it is mature enough to produce adequate seed.\Question: Choose the right answer from options given a) five b) seven c) six d) four
[A]: a


[Q]: Some sources claim there are more like ____ million vertebrate animals in zoos.\Question: Choose the right answer from options given a) six b) five c) one d) three
[A]: b


[Q]: Mothers have a baby only once every ____ or five years.\Question: Choose the right answer from options given a) four b) nine c) five d) eight
[A]:
a