Who suffered the most casualties in World War 1?

Speaking as a student studying this material, the textbook The American Promise: A History of the United States, (third edition) states that Germany suffered the most casualties, 2.2 million. Then Russia, with 1.9 casualties, 1.4 French, 900,000 British.

It is definitely Germany according to The American Promise: A Compact History Vol2 (fourth edition).