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.