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: Songbirds have ____ sets of vocal chords.\Question: Choose the right answer from options given a) six b) two c) eight d) ten

A: b
****
Q: Earwigs produce ____ or two generations every year.\Question: Choose the right answer from options given a) two b) no c) one d) nine

A: c
****
Q: Some experts believe whining peaks around the ages of ____ or four.\Question: Choose the right answer from options given a) three b) no c) four d) seven

A:
a
****