18. The power dissipated in a wire carrying a constant electric current I may be written as a function of I, the length l of the wire, the diameter d of the wire, and the resistivity ρ of the material in the wire. In this expression, the power dissipated is directly proportional to which of the following?
A) l only
B) d only
C) l and ρ only
D) d and ρ only
E) l, d, and ρ