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]: Ozone is a naturally occurring molecule containing ____ atoms of oxygen.\Question: Choose the right answer from options given a) three b) five c) nine d) eight
[EX A]: a

[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]: Aridity presents ____ main obstacles to solitary behavior.\Question: Choose the right answer from options given a) no b) three c) two d) five
[EX A]:
c