More Than Half a Century Ago, Two Cell Biologists Published

More than half a century ago, two cell biologists published details of their research involving isolated membrane vesicles from chloroplasts. Normally, the fluid inside these vesicles is near neutral. In an experiment, these membrane vesicles were soaked in an acidic solution (pH 4) until the inside of the vesicle also became pH 4. Based on the details provided in this scenario, by what mechanism could hydrogen ions have crossed the vesicle membrane, and what do you think happened at the molecular level?

A) Facilitated diffusion; hydrogen ions moved into the vesicle through a channel.
B) Osmosis; water moved from inside the vesicle to outside.
C) Simple diffusion; hydrogen ions moved into the vesicle through a channel.
D) Active transport; hydrogen ions moved out of the vesicle with the help of ATP.
E) Pinocytosis; water moved from outside the vesicle to inside.