An amoeba is a single-celled organism that feeds on algae, plant cells, and bacteria. When an amoeba feeds, it first makes contact with the food particle. Next, the amoeba starts to surround the particle. Once the particle is completely engulfed, it is surrounded by a layer of the amoeba's membrane, forming a vesicle. The pH in the vesicle is then lowered and the contents are degraded. What type of transport is involved in this process?

A. passive transport
B. diffusion
C. exocytosis
D. endocytosis