Four cylindrical wires are made of the same metal and are connected separately to identical ideal batteries that supply the same potential difference. The dimensions of each wire are as follows:
I. length L, radius r
II. length 2L, radius r
III. length L, radius 2r
IV. length 0.5L, radius 0.5r
Which wire carries the greatest current?
a) II only
b) All four wires carry the same current
c) III only
d) IV only