The rank of the matrix $A =\begin{bmatrix}1&2&3\\3&6&9\\1&2&3\end{bmatrix}$, is
Answer & explanation
Correct answer: option 1
We observe that $|A|=0$ and every square sub-matrix of order 2 is singular. But, A is a non-null matrix. Therefore, $r (A) = 1$.