Choose the correct option regarding cell structure and components:

• A
  $a-$ Some mature cells,$e.g.$,erythrocytes of many mammals,lack a nucleus.
  $b-$ Sieve tube cells of vascular plants have a nucleus.
• B
  $a-$ Centrioles are surrounded by a cell membrane.
  $b-$ Pairs of microtubules are present in a centriole.
• C
  $a-$ Cytoskeleton contains microfilaments made up of protein.
  $b-$ $70S$ ribosomes are present in chloroplasts.
• D
  $a-$ The color of plants is due to aleuroplasts.
  $b-$ Vacuoles can occupy up to $90\%$ of the volume of the animal cell.