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').

Mammals have only ____ sets of teeth.\Question: Choose the right answer from options given a) six b) five c) seven d) two
d

Most gerbils live three to ____ years and have few health problems.\Question: Choose the right answer from options given a) three b) four c) seven d) zero
b

Most amphibians reach maturity at three or ____ years.\Question: Choose the right answer from options given a) four b) six c) five d) two

a