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]: Carbon forms ____ bonds.\Question: Choose the right answer from options given a) five b) four c) two d) three
[EX A]: b

[EX Q]: Ozone is a molecule containing ____ oxygen atoms.\Question: Choose the right answer from options given a) one b) nine c) no d) three
[EX A]: d

[EX Q]: T cells serve ____ roles.\Question: Choose the right answer from options given a) two b) six c) seven d) nine
[EX A]:
a