The endoplasmic reticulum is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum, and smooth endoplasmic reticulum. It usally has ribosomes attatched to it.
Answer:a
Explanation: because naturally it occurs anytime it wants to
I am not sure but here is a link to help you :)
https://www.ncbi.nlm.nih.gov/pubmed/22322095