The main difference between chlorophyll $a$ and $b$ is:

• A
  Chlorophyll $a$ is a linear chain compound and $b$ is a branched chain.
• B
  Chlorophyll $a$ has no $Mg^{2+}$ ion in the center of the molecule.
• C
  In chlorophyll $a$ there is a $-CH_3$ group,whereas in $b$ it is a $-CHO$ group.
• D
  All of the above.