Is “Dentists all over the world are telling their patients the same things. You must floss regularly” an example of Ethos and why

No. It isn't an example of ethos.

Explanation:

Ethos refers to an argument that's based on the credibility of the person that's making the argument.

Ethos simply means establishing the authority to speak on a particular object subject. It's the persuasion and the argument whereby the knowledge, good moral character and the credibility of the speaker is shown.

In this case, the example given isn't an example of ethos because dentists in this case is a broad term and therefore does not give credibility from a particular dentist or probably a dental research.

sing because that is what they are doing in the sentence

explanation:

the simple predicate is the main verb in the predicate that tells what the subject does. example: my father fixed the dryer. read each sentence. draw a line under the complete predicate.