What is the best explanation for why digital data is represented in computers in binary?

A computer uses switches that are either on or off to send and receive signals. Since binary has two states (much like a switch) a computer can use and understand binary. Other number bases (such as denary) would need to be converted to binary in order for the computer to use/store/understand it.

By checking for voltage with a neon tester;

The steeply sloping region of a continental margin where the sea floor descends at a rate of greater than 1 ft for every 40 feet is called. continental slope.