我国技术标准规定电力系统的额定工作频率是（）Hz。
A.40
B.50
C.60
参考答案：B
The speech by the headmaster given on my graduation ceremony is strongly impressed ___ my memory.
A．to
B．over
C．by
D．on
肾上腺皮质分哪几个区域带？各带分泌哪些激素？
