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) Tumor cells are resistant to endostatin.
B) Endothelial cells are more responsive to chemotherapy.
C) Endostatin selectively targets endothelial cells.
D) Chemotherapy has minimal impact on endothelial cells.