Final answer:
Poirot suggests that the possibility of someone being in the room with Ackroyd when he died is eliminated because the murderer could have used a secret passage.
Step-by-step explanation:
Poirot suggests that no one was in the room with Ackroyd when he died. He explains that this could have happened through a secret passage. A secret passage is a concealed opening that allows someone to move from one area to another undetected. In the case of Ackroyd's death, Poirot proposes that the murderer used a secret passage to enter and exit the room without being seen.
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