Read the passage and answer the questions:Nature has so designed us that we are compelled to spend at least eight hours out of twenty four with eyes shut in sleep or in an attempt to sleep. It is a compensatory arrangement, perhaps for the strain the visual faculty undergoes during our waking hours, owing to the glut of images impinging upon it from morning till night. One who seeks serenity should, I suppose, voluntarily restrict one's range of vision. For it is mostly through the eye that the mind is strained or distributed. Man sees more than what is necessary or good for him. If one does not control one's vision, nature will do it for one sooner or later.The main theme of the passage is

• A. the need to sleep eight hours a day
• B. ways to reduce mental strain
• C. the connection between visual images and strain
• D. the necessity of controlling one's range