Answer:
a) As a general rule, direct current is used to refer to the current that flows in just one direction, and alternating current is used to refer to the current that flows in both directions at the same time at equal intervals of time.
b) The following are examples of DC sources: dry cell and automobile battery
AC generators and bicycle dynamos are two examples of sources of alternating current.
c) One significant advantage of alternating current over direct current is that it is considerably easier to transmit alternating current across large distances with little loss of electrical energy.
d) The frequency is 50 hertz.