For quotient rings "R"/"I", the change of rings is also very clear. An "R"-module is an "R"/"I"-module precisely when it is annihilated by "I". The submodule ann"I"("M") = { "m" in "M" : "im" = 0 for all "i" in "I" } is a left submodule of the left "R"-module "M", and is the largest submodule of "M" that is an "R"/"I"-module. If "M" is an injective left "R"-module, then ann"I"("M") is an injective left "R"/"I"-module. Applying this to "R"=Z, "I"="n"Z and "M"=Q/Z, one gets the familiar fact that Z/"n"Z is injective as a module over itself. While it is easy to convert injective "R"-modules into injective "R"/"I"-modules, this process does not convert injective "R"-resolutions into injective "R"/"I"-resolutions, and the homology of the resulting complex is one of the early and fundamental areas of study of relative homological algebra.