The correct answer is C. Both 1 and 2.
A chalcogenide glass is a glass containing sulphur, selenium or tellurium. Chalcogenide glasses are used in a variety of applications, including optical fibers, solar cells, and re-writable CD and DVD solid-state memory technology.
In re-writable CD and DVD solid-state memory technology, a chalcogenide glass is used as the recording medium. The glass is heated to a high temperature, and then a laser is used to write data onto the surface of the glass. The data is stored in the form of tiny pits and bumps on the surface of the glass. To read the data, the glass is heated again, and then a laser is used to read the data from the surface of the glass.
Chalcogenide glasses are a good choice for re-writable CD and DVD solid-state memory technology because they are relatively inexpensive, they have a high optical density, and they are stable over a wide range of temperatures.