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

[EX Q]: Digital photography is about ____ years away from being able to replace film photography.\Question: Choose the right answer from options given a) one b) no c) ten d) three
[EX A]:
d