How does Twain's decision to insert his own writing between passages from Susy's Biography affect the meaning of the passages from Susy's Biography? It helps to clarify times when Susy's descriptions are not accurate.

It serves to make Susy's writing better and of a higher literary quality.

It provides additional information to help understand the context of Susy's writing.

It provides a description of points at which corrections have been made to Susy's writing.