A Carnot Heat Engine Runs Between a Cold Reservoir at Temperature

Multiple Choice

A Carnot heat engine runs between a cold reservoir at temperature T_L and a hot reservoir at temperature T_H.You want to increase its efficiency.Of the following, which change results in the greatest increase in efficiency? The value of $\Delta$ T is the same for all changes.

A) Raise the temperature of the hot reservoir by $\Delta$ T
B) Raise the temperature of the cold reservoir by $\Delta$ T
C) Lower the temperature of the hot reservoir by $\Delta$ T
D) Lower the temperature of the cold reservoir by $\Delta$ T
E) Lower the temperature of the hot reservoir by (1/2) $\Delta$ T and raise the temperature of the cold reservoir by (1/2) $\Delta$ T