The answer is variant A because any other variant is a plural noun.
sg pl pane panes child children window windows
The correct answer is A. glass because the other words are plural, meaning they are talking about more than one.
The singular noun in the sentence is 'glass,' as it refers to one material. The other choices are plural nouns: 'children,' 'panes,' and 'windows.' Thus, the correct answer is option A: glass.
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