Direction: In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blanks out of the given alternatives.

Ask researchers this question, and listen as, like clockwork, a sense of awe and frustration creeps into their voices. In a way, it’s startling how universal sleep is: In the midst of the hurried ___(1)___ for survival, across aeons of bloodshed and death and flight, uncountable millions of living things have laid themselves down for a nice, long bout of unconsciousness. This hardly seems ___(2)___ to live to fight another day. “It’s crazy, but there you are,” says Tarja Porkka-Heiskanen of the University of Helsinki, a leading sleep biologist. That such a risky habit is so common and so persistent, suggests that whatever is happening is of the utmost importance. Whatever sleep gives to the sleeper is worth tempting death over and over again, for a lifetime. The precise benefits of sleep are still ___(3)___, and for many biologists, the unknowns are ___(4)___. One rainy evening in Tsukuba, a group of institute scientists gathered at an izakaya bar manage to hold off only half an hour before sleep is once again the focus of their conversation. Even simple jellyfish have to rest longer after being forced to stay up, one researcher marvels, referring to a new paper where the little creatures were nudged repeatedly with jets of water to keep them from drifting off. And the work on pigeons—have you read the work on pigeons? another asks. There is something fascinating going on there, the researchers agree. On the table, dishes of vegetable and seafood tempura sit cooling, forgotten in the face of these ___(5)___.