Copper brushes in D.C. machine are used
(a) where low voltage and high currents are involved
(b) where high voltage and small cur-rents are involved
(c) in both of the above cases
(d) in none of the above cases
(a) where low voltage and high currents are involved
(b) where high voltage and small cur-rents are involved
(c) in both of the above cases
(d) in none of the above cases