Which of the following metals has an $hcp$ crystal structure?

The arrangement $ABC ABC ABC …..$ is referred to as:

What is the number of octahedral and tetrahedral voids present respectively in $0.25 \ mol$ of a substance having $hcp$ structure?

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 .......

In a $A_x B_y$ crystal structure,$A^{+y}$ ions occupy all the tetrahedral voids and $B^{-x}$ ions form a $BCC$ unit cell. What is the formula of the compound?

What is the percentage of empty space in a $bcc$ unit cell (in $\%$)?