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').

Stable oxygen has eight protons and ____ neutrons.\Question: Choose the right answer from options given a) eight b) ten c) three d) zero
a

Ostriches have ____ stomachs.\Question: Choose the right answer from options given a) four b) nine c) eight d) three
d

Cows breed at ____ years, bulls at four.\Question: Choose the right answer from options given a) two b) three c) seven d) one

b