Energy costs vary greatly with energy source: coal @ $35.00/ton, gasoline @ a pump price of $2.75/gal, and electricity @ $0.100/kW·h. Conventional practice is to put these on a common basis by expressing them in $⋅GJ⁻¹. For this purpose, assume gross heating values of 29 MJ⋅kg⁻¹ for coal and 37 GJ⋅m⁻³ for gasoline.

(a). Rank order the three energy sources with respect to energy cost in $⋅GJ⁻¹. 

(b). Explain the large disparity in the numerical results of part (a). Discuss the advantages and disadvantages of the three energy sources.