The three substances below show these properties. • $X$ carries electric current in the solid state and also after melting. • $Y$ dissolves in water, and the resulting solution carries electric current. • $Z$ carries electric current only after it has been melted. What are $X$, $Y$ and $Z$?
- A$X$ Ca, $Y$ MgO, $Z$ NaOH
- B$X$ Ca, $Y$ NaOH, $Z$ MgO
- C$X$ MgO, $Y$ Ca, $Z$ NaOH
- D$X$ MgO, $Y$ NaOH, $Z$ Ca