下列物质中，属于单质的是请选择DCBA+l6r'y'v8L8]2t0^;W2g-@9a.J1S*y9i1x.`(r)d
A．MgO0^;W2g-@9a.J1S*y9i1x.`(r)d
B．MgCl20^;W2g-@9a.J1S*y9i1x.`(r)d
C．Mg0^;W2g-@9a.J1S*y9i1x.`(r)d
D．MgCO30^;W2g-@9a.J1S*y9i1x.`(r)d