Question

read the passage.
ac vs. dc
in the 1880s, the war of the currents began. thomas edison developed direct current, or dc, a type of electricity that flows in one direction. by 1887, dc was in use around the united states. it had some drawbacks, though. converting it to lower or higher voltages was difficult; that is, the currents force couldnt be changed easily. transmitted at the low voltage needed for use in homes and businesses, it lost power after traveling only a mile. meanwhile, nikola tesla was working with alternating current, or ac. ac reverses direction many times per second and can be changed to different voltages. at high voltage, it can be transmitted efficiently, traveling hundreds of miles with little energy loss. because of its advantages, ac triumphed by the end of the nineteenth century.
what is the main, or central, idea of the passage?
ac has distinct advantages over dc, so ac won the war of the currents.
ac reverses direction and can be converted to lower or higher voltage.

Explanation:

The passage first introduces DC (developed by Edison) with its drawbacks (difficult voltage conversion, power loss over short distance). Then it presents AC (worked on by Tesla) with its advantages (reverses direction, easy voltage change, efficient long - distance transmission). The conclusion “Because of its advantages, AC triumphed by the end of the nineteenth century” ties the advantages of AC to its victory over DC in the “War of the Currents”. The second option only mentions some features of AC without connecting it to the overall outcome (AC winning) which is the main focus of the passage.

Answer:

AC has distinct advantages over DC, so AC won the War of the Currents.