Suppose you have a sequence of key vectors represented as rows in a matrix, where $\mathbf{k}_0 = [1, 2]$, $\mathbf{k}_1 = [3, 4]$, $\mathbf{k}_2 = [5, 6]$, $\mathbf{k}_3 = [7, 8]$, and $\mathbf{k}_4 = [9, 10]$. Given a processing step at index $i=3$ and a context window size of $n_c=3$, which matrix is constructed by selecting the contiguous block of key vectors ending at the current step?