State the keys that will unlock the door to responsible service and selling.

The correct answer is C: Know the law, Know the customers, Know how and when to say, "No".

This answer is correct because it emphasizes three critical aspects of responsible service and selling in the context of bartending. Knowing the law ensures that you are compliant with alcohol serving regulations and can prevent legal issues. Knowing the customers allows you to better understand their needs and prevent over-serving. Knowing how and when to say, "No" is crucial in situations where serving more alcohol would be unsafe or irresponsible. These keys are essential for maintaining a safe and responsible environment for both customers and the establishment.

Options A, B, and D are incorrect as they do not address the key components necessary for responsible service and selling in a bartending setting.