A full account of the complexities of sleep, sought after by scientists, philosophers, and mystics for millennia,continues to elude us. That we are still so ignorant about a topic so (i) _________ to our daily lives is at once fascinating and (ii) _________.

Blank i	Blank ii
a) mysterious	d) deeply humbling
b) obscure	e) fully impenetrable
c) pertinent	f) totally blatant

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"Philosophers, scientists and Mystics"-none of them have been able to make a full account of the complexities of sleep hence it is fascinating that people of such caliber have not been able to find all the information yet it also puts a question mark on their capabilities, or pride that comes with their status and hence makes deeply humbling a proper contrast to fascinating.
We can also get rid of "fully impenetrable" because yes none of the above mentioned type of people have been able to find all about sleep but this does not mean that the task is fully impenetrable. Totally blatant is not coherent with the sentence, it does not establish any contrast.
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This is my response to the question and may be incorrect. Feel free to rectify any mistakes