it is truE thAt thErE ArE still ABunDAnt rEsErvEs oF mEtAls whiCh hAvE not yEt BEEn______, But it is nECEssAry to tAkE stEps to EnsurE thAt AltErnAtivE mAtEriAls ArE suBstitutED whEnEvEr possiBlE.
A．utilizED
B．moBilizED
C．ACquirED
D．CultivAtED