为了便于平稳控制循环氢脱塔底液位采用以下控制方法（）。
A、分程
B、串级
C、比值
D、单回路
参考答案：B
If human beings had been a bit less greedy and cruel, more birds and animals _____dying out.
A．ought to avoid
B．could have been avoided
C．should have avoided
D．might have avoided
用下列的（）快捷键可以打开“文件”菜单。
A、Ctrl+F
B、Ctrl+L
C、Alt+F
D、Alt+L
