conjunctions - "The ebb and flow of the tides ARE (IS?) now understood" - compound subject and verb coordination

Correct the sentence:

The ebb and flow of the tides / are / now understood.

In the above sentence I can go by two ways:

1. First is that according to are I should be using the before flow thus making ebb and flow as the two different entities.


2. Second is that since it is given as The ebb and flow , I can consider it as a single entity and use is in place of are.

Or is it that since tides is given , I need to go with are. I'm quite confused. Suggestions please.