单项选择题
A.proportional band
B.the error signal
C.the reset signal
D.feedback
单项选择题 Which of the listed statements is correct concerning refrigeration systems?（）
单项选择题 By which of the following means is force efficiently provided to vary the pitch of the blades on a modem controllable pitch propeller?（）
单项选择题 Which of the following statements concerning energy is correct?（）