Endostatin Is a 184-Amino Acid Protein That Only Affects Endothelial

Endostatin is a 184-amino acid protein that only affects endothelial cells.What is the best explanation for this observation?

A) Endostatin enters endothelial cells by endocytosis.
B) Endostatin can diffuse across the membrane of endothelial cells.
C) Endothelial cells have a receptor on their cell membrane that binds endostatin.
D) Only endothelial cells are exposed to endostatin.
E) Endothelial cells produce endostatin.