How many people were involved in the experiment giving rise to radioactivity

A．To warn people of the dangers of atomic power.
B．To describe the heroism of Louis Slotin.
C．To make a statement about what morality means.
D．To express the author's view of a scientist's duty.

A．the screwdriver slipped
B．a chain reaction began
C．he apologized to his co-workers
D．he calmly handled the accident