Final answer:
The most effective sentence combination that uses correct punctuation is: Rachel met a singer who lives in her apartment building.
Step-by-step explanation:
The most effective sentence combination that uses correct punctuation is:
Rachel met a singer who lives in her apartment building.
This sentence combines the information about Rachel meeting a singer and the fact that the singer lives in Rachel's apartment building. It uses the relative pronoun 'who' to connect the two pieces of information. This sentence is clear, concise, and grammatically correct.
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