A rule issued by the federal Environmental Protection Agency $($EPA$)$ limits the amount of carbon that can be emitted from a car's exhaust system. California state law prescribes much higher limits$.$ Under the U$.$S$.$ Constitution

a$.$ the California law takes precedence.

b$.$ both the rule and the law apply concurrently.

c$.$ the EPA rule takes precedence.

d$.$ both the rule and the law are invalid.