a$)$ Sketch the crystallographic unit cell for Face Centered Cubic $($FCC$)$ and Body Centered Cubic $($BCC$)$ crystal system. $(5$ point$)$

b$)$ Define atomic packing factor. $(4$ point$)$

c$)$ Briefly define the following terms; crystalline material, single crystal, polycrystalline material, amorphous, polymorphism, allotropy, anisotropy, isotropy. $(16$ point$)$