Question

Which provides the BEST description of the underlined phrase?

Answer 1

The answer is:  [C]:
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    "a gerund phrase that functions as the subject of the sentence". 
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        The phrase, "Walking my dog", begins with a "gerund" (ending in "-ing"  functioning as a "noun" or "subject of the sentence.
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Answer 2

C) a gerund phrase that functions as the subject of the sentence