Two dimensional close packed structure can be generated by stacking the rows of close packed spheres. This can be done in two different ways.
(I) The second row may be placed in contact with the first one, such that the spheres of the second row are exactly above those of first row. The spheres of the two rows are aligned vertically as well as horizontally.
If we call the first row as 'A' type of row, the second row being exactly same as the first one is also of 'A' type. Similarly, we may place more rows to obtain AAAA.... type arrangement
(II) In this type, the second row may be placed above the first one in a staggered manner such that its spheres. fit in depressions of first row. if the arrangement of spheres in the first row is called 'A' type the one in the second row is different and may be called 'B' type. When the third row is placed adjacent to the second in staggered manner, its spheres are aligned with those of first layer. Hence this layer is also 'A' type. The spheres of similarly placed fourth row will be aligned with those of the second row ('B' type). Hence this arrangement is of ABAB.... type
The type of voids generated in type (I) & (II) respectively are

Options

A. tetrahedral, octahedral
B. octahedral, tetrahedral
C. triangular, square
D. square, triangular

Answer

C