Here's why:
* Comma after introductory phrase: The phrase "please be quiet" acts as an introductory phrase. A comma is generally used to separate introductory phrases from the main clause.
* Conjunctive adverb: The word "and" is a coordinating conjunction that joins two independent clauses. Commas are typically used before coordinating conjunctions when joining independent clauses.
Correct Sentence: Please be quiet, and you are being too noisy.