Fill in the blanks:
$(a)$ In $ccp$ structure,$a = \dots \dots r$.
$(b)$ In $hcp$ structure,packing efficiency $= \dots \dots \%$.
$(c)$ In $bcc$ structure,percentage of packing efficiency $= \dots \dots \%$.

(N/A) In a $ccp$ (cubic close-packed) structure,the relationship between edge length $a$ and atomic radius $r$ is $a = 2\sqrt{2}r$.
$(b)$ The packing efficiency of an $hcp$ (hexagonal close-packed) structure is $74\%$.
$(c)$ The packing efficiency of a $bcc$ (body-centered cubic) structure is $68\%$.