I am solving a question in Ring and Modules. The question is as follows: $R$ is commutative ring with identity.
If $R[x]$ is PID then $R$ is a field.
I have solved the problem by considering the quotient $R[x]/(x) $.
But I found that in the exercise it is instructed to solve the problem considering.... "$R$ is PID iff every submodule of $R$ is free".
I have no idea. Can you please help me. Any hint will be welcomed. Thanks.