Congressional hearings determined that the use of mechanical power hammers is very dangerous to the persons using them and to persons in the vicinity of the persons using them. As a result, Congress enacted a statute prohibiting the use of mechanical power hammers on all construction projects in the United States. Subsequently, a study conducted by a private research firm concluded that nails driven by mechanical power hammers have longer-lasting joining power than hand-driven nails. After learning about this study, the city council of the city of Green enacted an amendment to its building safety code requiring the use of mechanical power hammers in the construction of all buildings intended for human habitation. This amendment to the city of Green's building safety code is 
unconstitutional, because it was enacted subsequent to the federal statute.  unconstitutional, because it conflicts with the provisions of the federal statute.  constitutional, because the federal statute does not expressly indicate that it supersedes inconsistent state or local laws.  constitutional, because the long-term safety of human habitations justifies some additional risk to the people engaged in their construction. 