According to the passage, what was used by sleep researchers

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

A．they feel they must follow an adult's orders
B．it is presented in connection with entertainment
C．they do not know that the information is going to be read by someone
D．due to their inability to distinguish an advertisement from a non-advertisement