氢火焰检测器主要由（）部分组成。
A、喷嘴
B、收集极
C、发射极
D、甲烷转化器
参考答案：A, B, C
——Is this the first time you _______ Qingdao?
—— No, but the first time I _______ here, the city wasn’t so beautiful.
A．visited; came
B．visited; have come
C．have visited; have come
D．have visited, came
合同授权在系统中哪个阶段可以操作？
