当平均产量达到最大值时（）。
A、总产量达到最大值
B、总产量仍处于上升阶段，还未达到最大值
C、边际产量达到最大值
D、边际产量等于零
参考答案：B
150. He was never late for school, _______ more than once by the class teacher..
A．being warned
B．after warned
C．having been warned
D．for warned
锅炉点火后应维持空冷散热器内较高背压，投入旁路前再根据低旁要求将机组背压降低到要求的数值（）Kpa。
