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

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.

A．the Civil War had eliminated racial discrimination in the U. S.
B．his grandmother could recite the history of the western world
C．Du Bois was considered something of a prodigy and very promising
D．his great-grandfather had fought in the American war of independence