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]: Natural chlorine contains ____ isotopes.\Question: Choose the right answer from options given a) two b) six c) eight d) seven
[A]: a


[Q]: Cleavage occurs typically in either one, two, three, four, or ____ directions.\Question: Choose the right answer from options given a) six b) three c) ten d) two
[A]: a


[Q]: Ligand seeks ____ receptors into binding and mutual phosphorylation.\Question: Choose the right answer from options given a) seven b) three c) five d) two
[A]:
d