21 In a cubic close-packed array of N spheres thereare:(a) N tetrahedral interstices.(b) 2N tetrahedral interstices.(c) 4N tetrahedral interstices.22 In a hexagonal close-packed array of N spheresthere are:(a) N octahedral interstices.(b) 2N octahedral interstices.(c) 4N octahedral interstices.23 A tetrahedron is a polyhedron with:(a) Four triangular faces.(b) Six triangular faces.(c) Eight triangular faces.24 An octahedron is a polyhedron with:(a) Four triangular faces.(b) Six triangular faces.(c) Eight triangular faces.
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