Why did warfare in the 20th century see enormously high casualties as compared to previous wars？

A The casualty rate rose during the 20th century because new technology made war far deadlier.
B The casualty rate rose during the 20th century because soldiers fought harder.
C The casualty rate rose during the 20th century because fighters were no longer protected by tanks.
The casualty rate rose during the D 20th century because leaders saw soldiers as replaceable.