Question

looking into glass
heat two ordinary materials - sand and a salt - at extremely high temperatures, and what results is a fabulous, mysterious, liquid substance that is resistant to heat, cold, and high winds. it can contain water and withstand strong pressure. glass is one of the crowning jewels of human invention.
no one knows for sure when glass was first produced. some archeologists have traced the earliest examples back to at least 4,000 years ago, in a middle eastern area once known as mesopotamia.
which statement is a main idea of the passage?
historians are unsure when glass was first created.
the development of glass lenses led to many important inventions.
early versions of glass were impossible to see through.
minerals from saltwort are the most important ingredient in glass.

Explanation:

The passage mainly discusses glass - its creation from sand and salt, properties, and uncertainty about its origin. The main idea is that historians are unsure when glass was first created.

Answer:

Historians are unsure when glass was first created.