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]: Ocular dryness responds to the use of artificial tears applied every ____ to three hours.\Question: Choose the right answer from options given a) no b) one c) three d) eight
[EX A]: b

[EX Q]: Many tortoises can retract their head, their ____ limbs and tail into the shell for protection.\Question: Choose the right answer from options given a) no b) one c) two d) four
[EX A]: d

[EX Q]: Water emits oscillations that are measurable from ____ through Megahertz.\Question: Choose the right answer from options given a) two b) zero c) no d) ten
[EX A]:
b