引导信号应在列车头部越过()后,及时关闭。
参考答案:信号机
--Jassy , you speak Chinese so fluently!
--- ______.
You are lying, I have to say. B. Thanks a lot for saying so.
C. That’s none of your business. D. I can’t agree more.
在酸性条件下,草酸钠和高锰酸钾的()温度应保持在()℃,所以滴定操作必须()进行,若溶液()过低,需适当()。