If a language is contained in other langauge, is it of the same complexity? If some language $L$ is in P, and some other language $K$ is contained in $L$, does that mean that $K$ is also in P?

Thanks :)
No. $\Sigma^*\in\mathbf{P}$ and every langauge is a subset of that&nbsp;&ndash; even undecidable ones.