Which sources produce alternating current?

State the principle of an electric generator.

Name some sources of direct current.

Choose the correct option:

A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced cum changes once in each:

(a) two revolutions

(b) one revolution

(c) half revolution

(d) one-fourth revolution

Name two safety measures commonly use in electric circuits and appliances.