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.

A．Because he sets up a model of international radiation safety standards.
B．Because he put forward that the radiation produced by all the mobile phones in the enclosed spaces will bounce around and be harmful.
C．Because the commuters worried that the metal walls of the railway carriage will threaten their health.
D．Because the commuters are afraid that the use of mobile phone will be banned in the railway carriage.