Semiring identities of semigroups of reflexive relations and upper triangular boolean matrices

We show that the following semirings satisfy the same identities: the semiring $\mathcal{R}_n$ of all reflexive binary relations on a set with $n$ elements, the semiring $\mathcal{U}_n$ of all $n\times n$ upper triangular matrices over the boolean semiring, the semiring $\mathcal{C}_n$ of all order preserving and extensive transformations of a chain with $n$ elements. In view of the result of Klíma and Polák, which states that $\mathcal{C}_n$ has a finite basis of identities for all $n$, this implies that the identities of $\mathcal{R}_n$ and $\mathcal{U}_n$ admit a finite basis as well.