I would say aggressive, because you can’t really use the other words without being negative.
<span>For over two hundred years, the reason for New England's Dark Day was unknown.
This represents the main idea because it lets the reader know that the passage is talking about a "Dark Day", where it happened (New England), and the fact that for over two hundred years it was a mystery. It also gives an indication that the mystery will have been solved by the end of the passage. </span>
