Which of the Following Is Not One of the Reasons

Which of the following is not one of the reasons why federal preemption of a state law generally occurs?

A) There is a literal conflict between the state and federal measures,so that it is impossible to follow both simultaneously.
B) The federal law specifically states that it will preempt state regulation in certain areas.
C) The federal law was enacted before the state law.
D) The federal regulation is persuasive.