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

A．can commit murder or other dangerous things
B．can be fatally hurt if they are wakened abruptly
C．are partially awake
D．are immune to injury

A．has lost so much sleep that he is now suffering from insomnia
B．has conducted experiments on sleepwalkers
C．has slept more than any other living man of his age
D．does not believe that there are any sleepwalkers at all