Every nth row matrix

Hello

I have a matrix, let's call it A. I want to define a new matrix by taking every n-th row in the A matrix. How do i do that?

For example,

A= | 2 3 5 |
     | 9 2 1 |
     | 5 1 2 |
     | 1 8 9 |

the B, if n=2, is equal to

| 2 3 5 |
| 5 1 2 |