What Prompted Psychiatrist Erich Lindemann to Describe Three Phases of the Grieving

What prompted psychiatrist Erich Lindemann to describe three phases of the grieving process?

A) Working with people who had lost loved ones in the 9/11 attacks on America
B) Working with people whose relatives had experienced hardships during the Great Depression
C) Working with people whose relatives had been killed in World War II
D) Working with people whose relatives had been killed in the Coconut Grove fire