What does the phrase "taken with a barrel of salt" mean at end of the second paragraph

A．sleepwalkers are often awakened by dangers
B．the underlying cause of sleepwalking is more serious than sleepwalking itself
C．most sleepwalkers are deranged or insane
D．all of the above

A．studied sleepwalking for at least ten years
B．concluded that sleepwalkers are partially asleep in their sensory area
C．maintained that it is a mistaken belief that sleepwalkers are immune to injury
D．A and B