The correct answer is: A. Ordinary glass.
Ordinary glass is a type of glass that is made from silica, soda ash, and lime. It is a common material that is used in a variety of applications, including windows, bottles, and lenses. Ordinary glass is relatively inexpensive and easy to work with, which makes it a popular choice for many applications.
Pyrex glass is a type of glass that is made from borosilicate, which is a type of glass that is resistant to thermal shock. Pyrex glass is often used in cookware and laboratory equipment.
Flint glass is a type of glass that is made from lead oxide. It is a type of glass that is known for its high refractive index, which makes it useful for making lenses.
Cobalt glass is a type of glass that is made from cobalt oxide. It is a type of glass that is known for its blue color.
In conclusion, the correct answer is: A. Ordinary glass.