An alloy of metals $A$,$B$,and $C$ is found to have $A$ constituting a $ccp$ lattice. If $B$ atoms occupy the edge centers and $C$ is present at the body center,the formula of the alloy is:

The $CORRECT$ statement$(s)$ for cubic close packed $(ccp)$ three dimensional structure is(are)
$(A)$ The number of the nearest neighbours of an atom present in the topmost layer is $12$
$(B)$ The efficiency of atom packing is $74 \%$
$(C)$ The number of octahedral and tetrahedral voids per atom are $1$ and $2$,respectively
$(D)$ The unit cell edge length is $2\sqrt{2}$ times the radius of the atom

Calculate the void volume in an $fcc$ unit cell if the total volume of the unit cell is $6.4 \times 10^{-23} \text{ cm}^3$.

Which of the following statements is not correct?

What is the coordination number of a one-dimensional close-packed arrangement?

When sodium is heated above $1000 \ K$,its crystal structure changes from $bcc$ to $fcc$. Assuming the metallic radius of the atom remains unchanged,the ratio of the fraction of volume occupied by atoms in $bcc$ and $fcc$ structures will be .......