# If $A$ and $B$ are any two matrices such that $AB=O$ and $A$ is non-singular, then

$\begin{array}{1 1}(1)B= O&(2)B\; is\; singular \\ (3)B\; is\; non-singular &(4)B=A \end{array}$

$B= O$
Hence 1 is the correct answer.