#### Answer

Statements A and C.

#### Work Step by Step

Use equation 15–5 to compute the upper limit on the efficiency.
$$e_{ideal}=1-\frac{T_{low}}{T_{high}} = \frac{T_{high}- T_{low}}{T_{high}}$$
Statement A, increasing the temperature difference, raises the theoretical efficiency.
The process described in statement C keeps the temperature difference the same and decreases the high temperature. This also raises the theoretical efficiency.