Plankton are organisms that float. Why is a copepod, which can swim and propel itself through the ...

Plankton are organisms that float. Why is a copepod, which can swim and propel itself through the water, classified as plankton?
 
  a) It lives at the surface of the ocean where plankton live.
  b) It was classified as a plankton before it was known that copepods are swimmers.
  c) It has a swim bladder that keeps it from sinking, so it is a floater.
  d) It spends part of its life floating and part swimming, so it is classified as plankton because of the floating part.
  e) Because of its small size, it cannot swim very fast or very far, so its location is determined by currents.