I know the answer is C, but don't know the procedure... thanks

91. Some metals are found in the free uncombined state while other metals are found in combined with other elements. The major reason is: (A) The more active metals can be found uncombined while the less active metals occur combined. (B) Metals with negative reduction potentials can be found uncombined while metals with positive reduction potentials are found combined. (C) Metals with positive reduction potentials can be found uncombined while metals with negative reduction potentials are be found uncombined combined. (D) There is no way we can predict which metals will be uncombined or combined. x