Let $A$ and $B$ be two square matrices of order 3 and $A B=O_{3}$, where $O_{3}$ denotes the null matrix of order 3. Then,

• A

  must be $A=O_{3}, B=O_{3}$
• B

  if $A \neq O_{3}$, must be $B \neq O$,
• C

  if $A=O_{3}$. must be $B \neq O_{3}$
• Correct answer
  D

  may be $A \neq O_{3} . B \neq O_{3}$

Hence, the correct answer is option: 'D'.