How Was the Positron Discovered

Multiple Choice

How was the positron discovered?

A) By creating them in very high energy collisions of heavy nuclei such as gold nuclei.
B) By observing the cloud chamber tracks of high energy gamma rays as they created positron- electron pairs.
C) By observing the cloud chamber tracks of cosmic rays as they passed through a metal plate.
D) By allowing protons to collide in a particle accelerator.
E) By allowing electrons to collide in a particle accelerator.