The ends of a metal rod are kept at temperature `theta_(1) and theta_(2)` with `theta_(2) gt theta_(1)`. At steady state the rate of flow of heat along the rod is directly proportional to
A. the length of the rod
B. the diameter of the rod
C. the cross-sectional area of the rod
D. the temperature difference `(theta_(2)-theta_1)` between the ends of the rod
