Which metals can be extracted by heating alone?

The correct choice highlights that mercury and silver can be extracted by the process of heating alone, particularly through methods such as roasting or smelting, which involve the application of heat to bring about the separation of the metals from their ores or compounds.

Mercury, for instance, can be extracted through the heating of its ore, cinnabar (mercury sulfide), which releases metallic mercury vapor when heated. Silver can also be liberated from its ores by direct heating, especially from ores like silver sulfide, which decomposes upon heating.

In contrast, iron and zinc typically require reduction with a carbon source or other methods in addition to heating for effective extraction. Lead and copper extraction often involves processes like smelting with specific reducing agents, which means that solely relying on heat is not sufficient for their extraction. Gold and platinum, being noble metals, often occur in their elemental forms and do not need extraction or refining through heating, making them distinct from the extraction processes applicable to others mentioned.

Thus, mercury and silver stand out as metals that can effectively be extracted using only heat in specific processes, making this the correct choice in the context of this question.