For example, it has long been known that total sleep （46）is 100 percent fatal to rats, yet, （47）examination of the dead bodies, the animals look completely normal. A researcher has now （48）the mystery of why the animals die. The rats （49）bacterial infections of the blood, （50）their immune systems -- the self-protecting mechanism against disease -- had crashed.
46.[A] reduction
[B] destruction
[C] deprivation
[D] restriction
47.[A] upon
[B] by
[C] through
[D] with
48.[A] paid attention to
[B] caught sight of
[C] laid emphasis on
[D] cast light on
49.[A] develop
[B] produce
[C] stimulate
[D] induce
50.[A] if
[B] as if
[C] only if
[D] if only
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