free metal 一問~

想請問most of the free metal we used are obtained from the _______________
(1) rocks (2) air (3) sea

A. (1) only B. (2) only C. (1) and (2) only D (2) and (3) only

答案係咩，點解~???同咩叫free metal???

thanks a lot~ 10 points will be given~