In the Section "Investigating Life: Cutting Off a Tumor's Supply

In the section "Investigating Life: Cutting Off a Tumor's Supply Lines in the War on Cancer",endostatin is a 184-amino acid protein that only affects endothelial cells.What is the best explanation for this observation?

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