Why did europeans generally buy slaves from slave traders rather than capturing the slaves themselves?

a slaves were so cheap that capturing them was not worth the effort to europeans.
b laws prohibited europeans from capturing slaves but allowed them to buy slaves.
c europeans were morally opposed to capturing slaves but not to buying them.
d africans were generally able to hide from or defend themselves against europeans.

world war i began in 1914, after the assassination of archduke franz ferdinand, and lasted until 1918. during the conflict, germany, austria-hungary, bulgaria and the ottoman empire (the central powers) fought against great britain, france, russia, italy, romania, japan and the united states (the allied powers). to new military technologies and the horrors of trench warfare, world war i saw unprecedented levels of carnage and destruction. by the time the war was over and the allied powers claimed victory, more than 16 million people—soldiers and civilians alike—were dead.

explanation:

i believe it was the shinto