Experiment.
It is a systematic process that carries out to make a finding or determine an identified fact, an inventive act or method. It is also authenticating a theory, offers perception into principle of causation by validating what conclusion arises when a specific aspect is handled
Answer:
Slaves who ran away toward Union troops were considered E. "contrabands of war."
Explanation:
Although some slaves were returned to their masters, most of them were kept in the soldiers’ lines and were allowed to help and participate in the war. They were called “contrabands of war”. These contrabands served as a big work force and proved themselves as extremely useful in the Civil War. By the end of it approximately 186,000 African American men had enlisted.
The answer to this question is B. Elevators
One of the reason why skyscrapers were not popular before the invention of elevators was that it took too much energy to move from the groud floor to the upper floor with the stairs alone.
After the invention of elevators, we eliminate this problem and people could move from ground floor to upper floor with only little effort.