The top layer looks as shown below. Mark the atoms that form a face of the unit cell cube by annotating the image below.

Question #$2$:

In which layer is the opposite face to the one marked in Question #$1,$ the bottom or the middle?

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Question #$3$:

How many marbles are involved in the making up of the unit cell? $($Count the marbles and don't divide them between cells$)$

Structure B

Question #$4$:

The top layer looks as shown below. Mark the atoms that form a face of the unit cell cube by annotating the image below. Look carefully and remember that all edges of a cube have the same length.