What does Scrooge do after the incident with the door
knocker?

He is visited by Marley's ghost

Explanation:

Instead of the door knocker, Scrooge sees Marley's face. Marley's face is not shadowed, like the rest of the objects in the yard, but is strangely illuminated. The expression on this disembodied face is neither angry nor violent; it looks just as Marley was apt to look when he was alive. Scrooge notes that the horrible specter of Marley's face rested on its otherworldly nature, not on its expression

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