Concavity/convexity of a function in a subset where the function is concave/convex

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Let $f:R^n \rightarrow R$. Suppose $f$ is concave(convex) in $A\subseteq R^n$, where $A$ is open and convex. Moreover let us consider $B\subseteq A$, closed. Is $f$ concave/convex in that set ? Is it necessery that $ B $ is convex as well? ?