第2196题：principal submatrices

If $A$ is any $n \times n$ matrix, let ${(r)} \atop A$ denote the $r \times r$ submatrix in the upper left corner of A, that is ${(r)} \atop A$ is the matrix obtained from $A$ by deleting the last $n-r$ rows and columns. the matrics ${(1)} \atop A$ , ${(2)} \atop A$ , ${(3)} \atop A$ , $\cdots$ , ${(n)} \atop A$ are called the principal submatrices of $A$ .

If $A= \begin{bmatrix} 1 & 7 & 4 & 1 \\ 2 & 2 & 1 & 5 \\ 3 & 1 & -1 & 6 \\ -2 & 4 & 3 & 1 \end{bmatrix}$  then ${(3)} \atop A$ =(  ).

A. $\begin{bmatrix} 1 & 7 & 1 \\ 2 & 2 & 5 \\ 3 & 1 & 6 \end{bmatrix}$

B. $\begin{bmatrix} 1 & 7 & 4 \\ 2 & 2 & 1 \\ 3 & 1 & -1 \end{bmatrix}$

C. $\begin{bmatrix} 7 & 4 & 1 \\ 2 & 1 & 5 \\ 1 & -1 & 6 \end{bmatrix}$

D. $\begin{bmatrix} 2 & 2 & 1 \\ 3 & 1 & -1 \\ -2 & 4 & 3 \end{bmatrix}$