for the subclasses ${mathcal m}_1$ and ${mathcal m}_2$ ofmonomorphisms in a concrete category $mathcal c$, if ${mathcalm}_2subseteq {mathcal m}_1$, then ${mathcal m}_1$-injectivityimplies ${mathcal m}_2$-injectivity. the baer type criterion is about the converse of this fact. in this paper, we apply injectivity to the classes of {it $c$-dense, $c$-closed} monomorphisms. ...
