Which of the following was a major reason the united states exchanged neutrality for war in 1917?
a. americans felt it was their duty to come to the aid of the central powers, who were being badly beaten.
b. the germans had taken over a vital trade route between britain and the united states.
c. the germans massacred the entire population of a major british city, including civilians.
d. the germans used their submarines to attack ships carrying american citizens and supplies to the allies.