(a) Resistance of a conductor depends directly on its length, inversely proportional to the area of cross-section, directly proportional to the temperature and it depends on the nature of the conductor.
(b) Metals have free electrons which can move and conduct electricity, whereas glass does not have free electrons which can flow freely to conduct electricity.
(c) The resistivity of an alloy is generally higher than that of pure metal. Alloys do not oxidise (burn) readily at higher temperatures. Therefore, conductors of electric heating devices, such as toasters and electric irons, are made of an alloy rather than pure metal.