单项选择题

Ability to sleep is largely a habit. The conditions referred to only lead to sleep. Out-of-door (31) , a good habit of regular drinking and the avoidance of late eating and (32) are all helpful to sound sleep. Such factors are largely within the (33) of any person. A bath at (34) , neither hot nor cold but of body temperature, may be helpful to sleep. Sleeping pills should never be taken except when suggested by (35) .

33（）

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