The correct answer to this open question is the following.
Unfortunately, you forgot to attach the statements to answer the question. Without those statements, we do not what the options are.
However, trying to help you, we did some research and can comment on the following.
There are estimated to have been over thirty-seven million military and civilian casualties in World War I. A statement that is NOT a primary reason for the high death toll would be "WWI saw the first use of firebombing in order to decimate cities which resulted in a high civilian casualty rate.
World War I was the deadliest war of all time in that moment of human history. There were various reasons for that. During the war, new technological advances appeared and killed millions of people: civilians and the military. We are talking about tanks, machine guns, and poison gas. These new weapons created so much pain, suffering, and destruction.
Let's have in mind that World War I was fought on a global scale including colonial efforts which made disease outbreaks a leading cause of death. Soldiers have to fight in and out of the trenches to capture territories. Those actions produced millions of dead soldiers.
