Answer:
(a) 
(b) 
Explanation:
(a) The surface current density of a conductor is the current flowing per unit length of the conductor.

Considering a wire, the current is uniformly distributed over the circumferenece of the wire.

The radius of the wire = a

The surface current density 
(b) The current density is inversely proportional
......(1)
k is the constant of proportionality

........(2)
substituting (1) into (2)





substitute 

Mass is a body of matter of indefinite shape and considerable size. Density is the degree to which something is filled. Check this website for more a better understanding : <span>www.answers.com/Q/What_is_the_relationship_between_density_and_volume</span>
The frequency of the radio station is

For radio waves (which are electromagnetic waves), the relationship between frequency f and wavelength

is

where c is the speed of light. Substituting the frequency of the radio station, we find the wavelength:
By putting this special transportation plastic on the bottom of the sled, because the transportation plastic is slick. that is what the transport bins slide around on. (p.s) the plastic is really expensive!