$($b$)$ Iron is a ferromagnetic material. However, an iron nail usually does not show ferromagnetic properties even below the Curie temperature. Why? What happens to its microscopic structure above the Curie temperature?

$($c$)$ "The dispersion $($frequency $$ vs$.$ wave$-$vector $k)$ relation of an elastic wave in a fluid is linear in $k.$ But it is not so in a solid in general" $-$ why? Why does the group velocity of an elastic wave propagating in a solid vanish at the Brillouin zone boundaries?