单项选择题
A.fire-tube controlled circulation
B.fire-tube supercritical circulation
C.water-tube natural circulation
D.water-tube express circulation
单项选择题 Which of the lists forms of heat transfer, if any, is illustrated by the flow of combustion gases through a boiler?（）
单项选择题 Auxiliary boilers can be classified as（）
单项选择题 When the tilting box of a variable stroke axial-piston pump is perpendicular to the pump shaft, which of the following conditions will exist?（）