Read the passage. The Northmen came to Paris with 700 sailing ships. This is not counting those of smaller size which are commonly called barques. At one stretch the Seine was lined with the vessels for more than two leagues. You could ask in astonishment in what cavern the river had been swallowed up. There were so many boats the river could not to be seen.

–Adapted from "Wars of Count Odo with the Northmen” by Abbo

What is the central idea of the passage?

Parisians were caught asking what cavern had swallowed up the river.
Smaller boats sailing on the Seine were commonly called barques.
The Northmen filled the Seine with so many boats that the river could not be seen.
The Northmen’s sailing vessels lined up along the Seine for more than two leagues.