Are you sure you weren't given any options? Basically the electrical components, e.g wires and bulbs will be conductors, and the part comprising the outer casing, e.g plastic, glass, will be insulators
The point of contact the path difference is zero but one of the interfering ray is reflected so the effective path difference becomes λ/2 thus the condition of minimum intensity is created in the center.
C., used in power plants I think.