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').

[Q]: Atoms are extremely small, on the order of a few ____ billionths of a meter.\Question: Choose the right answer from options given a) ten b) seven c) nine d) eight
[A]: a


[Q]: Sea stars have ____ arms, or rays, connected to a small round body.\Question: Choose the right answer from options given a) five b) six c) seven d) no
[A]: a


[Q]: Water is visualized as occupying the center of a tetrahedron, a solid with ____ faces.\Question: Choose the right answer from options given a) four b) ten c) seven d) zero
[A]:
a