$A$ mass $M$ hangs on a massless rod of length $l$ which rotates at a constant angular frequency $\omega$. The mass $M$ moves with steady speed in a circular path of constant radius $r$. The angular momentum of $M$ about point $A$ is $L_A$,which lies in the positive $z$-direction,and the angular momentum of $M$ about point $B$ is $L_B$. Which of the following statements is correct for this system?

• A
  $L_A$ and $L_B$ are both constant in magnitude and direction.
• B
  $L_B$ is constant in direction with varying magnitude.
• C
  $L_B$ is constant,both in magnitude and direction.
• D
  $L_A$ is constant,both in magnitude and direction.