下列图标表示（）电压等级。
A.1000kV
B.500kV
C.220kV
参考答案：A
当列车早晚点超过2分钟时，（）会发出提示，并自动给出列车的运行调整方案。
A、ATP系统
B、ATS系统
C、AFC系统
D、ATC系统
A stock priced at $100 has a 70 percent probability of moving up and a 30 percent probability of moving down. If it moves up, it increases by a factor of 1.02. If it moves down, it decreases by a factor of 1/1.02. What is the probability that the stock will be $100 after two successive periods（）
A. 9%.
B. 42%.
C. 21%.
