What's the main idea of the first paragraph
A．Not many elements occur in nature as pure substances, therefore ancient people wouldn't pick them up.
B．Ancient people found copper, iron, lead, tin and zinc very useful, although they did not consider these to be elements.
C．Ancient people discovered elements, found them free or lying around in their natural form and put them to use.
D．Sulfur and carbon have been used by mankind for thousands of years, but not much has been known about them.