What do $A$ and $B$ represent in the given figure?

• A
  $A = \text{Chromatid}, B = \text{Centromere}$
• B
  $A = \text{Chromosome}, B = \text{Centromere}$
• C
  $A = \text{Centromere}, B = \text{Arm}$
• D
  $A = \text{Kinetochore}, B = \text{Arm}$