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]: Heme is made of ____ pyrrole groups united to form a porphyrin ring.\Question: Choose the right answer from options given a) five b) four c) eight d) six
[EX A]: b

[EX Q]: Roundworms are fairly large worms that can grow to be ____ or five inches long.\Question: Choose the right answer from options given a) no b) nine c) four d) zero
[EX A]: c

[EX Q]: Marine mammals acquire water from ____ sources.\Question: Choose the right answer from options given a) two b) nine c) five d) three
[EX A]:
d