Key Matrix from a Sliding Window

Consider a matrix $\mathbf{M}$ that contains 10 row vectors, indexed from 1 to 10 (i.e., $\mathbf{m}_1, \mathbf{m}_2, ..., \mathbf{m}_{10}$). The notation $\mathbf{M}_{[a, b]}$ is used to select a sub-matrix by vertically stacking the row vectors from index $a$ to index $b$, inclusive. Which of the following sub-matrices correctly represents $\mathbf{M}_{[6, 9]}$?

The notation $\mathbf{M}_{[j-9, j]}$ is used to select a slice of row vectors from a larger matrix $\mathbf{M}$. This slice contains a total of ____ row vectors.

Extracting a Context Window from a Token Matrix