The mobilization of streetcars and subways were made possible by
a. electricity.
c. radio waves.
b. internal-combustion engines.
d. steel.

The answer is (a.) electricity. The mobilization of Streetcars and Subways were made possible by the electricity. The manufacturers thought that the electricity was very beneficial because, with the help of electricity, streetcars,and subway were electrified. The mobiles were manufactured quicker than it used to be.

Iwould say a is accurate

Sound energy is produced when an object vibrates. examples are 1- a balloon popping2-a whistle3- a waterfall