355. Which of the following methods is typically employed in the design of waste heat boilers to obtain maximum heat transfer, while maintaining low overall weight?

A. Feedwater is preheated in a separately fired economizer.
B. An external superheater unit is located above the boiler in the gas passages.
C. An unfired exhaust gas preheater is added to increase the heat transfer rate.
D. Steel fins are installed on the generating tube surfaces to increase the effective surface area.