A restrictive clause provides essential information to a sentence without the use of commas, such as identifying a specific noun. In the given sentences, 'The astronaut who first stepped on the moon was Neil Armstrong' contains a restrictive clause.
Restrictive clauses provide essential information to the sentence and are not set off by commas. In the context provided, the sentence that contains a restrictive clause is: The astronaut who first stepped on the moon was Neil Armstrong. The clause 'who first stepped on the moon' is restrictive as it identifies a specific astronaut.
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