The Khmer Rouge Used Executions, Starvation and Labor Camps to Subdue

The Khmer Rouge used executions, starvation and labor camps to subdue the people of Cambodia. What is the estimate of the numbers killed the Khmer Rouge campaign during its four year reign
On the 1970s?

A) 1 million
B) 3 million
C) 5 million
D) 2 million