This passage and the broader portrayal of memory it helps to develop inthe novel demonstrates a prescient understanding on Woolf's part of our currentconception of how human memory works.

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it代指的是什么啊，假如主語是This passage and the broader portrayal of memory 那樣demonstrates 為何是第三人稱單數？

這個句子應該有二個逗號：

This passage, and the broader portrayal of memoryit helps to develop in the novel, demonstrates a prescient understanding onWoolf's part of our current conception of how human memory works.

二個逗號之間為插入語，主語為單數名詞 this passage. 所以謂語動詞用單數形式。it 指代 this passage.