§ Tensoring with base ring has no effect

If 0MMM0 \rightarrow M' \rightarrow M \rightarrow M'' is an exact sequence of AA modules, then so is 0AMAMAM0 \rightarrow A \otimes M' \rightarrow A \otimes M \rightarrow A \otimes M''. (As an aside, the order is M,M,MM', M, M'' since the middle term MM is somehow put together from the other two. So MM is the primary. Then by human convention, we choose the single prime on the left and the double prime on the right).