In This post on mathoverflow, the OP states that ''It is known that if $R$ is a DVR with fraction field $K$, then the $R$-submodules of $K$ are $0,K,x^nR$, with $n$ any integer and $x$ a generator of the maximal ideal of $R$.
Can somebody please tell me where I can read about this statements. I tried to search for it in the Internet and my school's library but didn't find anything.
Thanks in advanced.