The sentence, ''The experiment showed that “light is faster than a speeding bullet'', is the one that expresses the use of a signal phrase. Therefore, the option A holds true.
The signal phrase can be referred to or considered as a phrase that expresses use of an outer source of information in any given passage. The aforementioned sentence shows a known information that is outer to the entire passage, but relates to its context.
Therefore, the option A holds true and states regarding the significance of a signal phrase.
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