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

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

A．There is no accurate figure of the number of sleepwalkers.
B．Stories of sleepwalkers are all fantasies.
C．Sleepwalkers can be considered half awake in their sleep.
D．Voltaire knew a sleepwalker who once danced a minuet in sleep.