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]: Metals often crystallize in a ____ dimensional version of the hexagonal closest packed structure.\Question: Choose the right answer from options given a) one b) seven c) three d) eight
[EX A]: c

[EX Q]: Echinoderms usually have ____ appendages, but there are some exceptions.\Question: Choose the right answer from options given a) ten b) five c) seven d) three
[EX A]: b

[EX Q]: Light drives the interconversion of ____ forms of phytochroms.\Question: Choose the right answer from options given a) zero b) one c) seven d) two
[EX A]:
d