Final answer:
The underlined sentence can be rephrased as 'Grandpop could only nod because he was too tired to speak.'
Step-by-step explanation:
The sentence that has the same meaning as 'Grandpop nodded to help keep the rhythm as he played the harmonica' is 'Grandpop could only nod because he was too tired to speak.' In both sentences, Grandpop is unable to speak and can only communicate through nodding. The reason for his inability to speak is different in each sentence. In the underlined sentence, Grandpop nods to maintain the rhythm while playing the harmonica.
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