Because of the Doctrine of Federal Supremacy

Because of the doctrine of federal supremacy,

A) no state may impose a regulation if there is an existing federal law to cover the problem.
B) no state law may impose a stricter standard than a federal law.
C) state statutes may impose standards that are stricter than federal laws unless the federal law is in an area where Congress wants there to be no state regulation.
D) both a and b.