All of the following Were suggested as being used by sleep researchers EXCEPT ______.

A．a gradual parting from the real world
B．undisturbed by tension or problems
C．drowning in an ocean of darkness
D．dependent only Upon individual age and health

A．after dreaming has stopped
B．when his EEG reveals no alpha rhythm
C．when his EEG begins to show an alpha rhythm
D．after the completion of his alpha rhythm decline