Ribosomes are sites where proteins are made.
E ndoplasmic reticulum are membranes attached to ribosomes.
Endoplasmic reticulum are membranes attached to sites where proteins are made.
What are membranes attached to sites where proteins are made?

  the body's largest organ
  Veins and arteries.
  epidermis and dermis
  layers of fat
  cell walls
  cell membranes
  endoplasmic reticulum
  brain cells