当电网中发生下列哪些故障时，采取相应措施后应能保证稳定运行，满足电力系统第二级安全稳
题型：多项选择题
问题：
当电网中发生下列哪些故障时，采取相应措施后应能保证稳定运行，满足电力系统第二级安全稳定标准？
A．向城区供电的500kV变电所中一台750MVA主变故障退出运行
B．220kV变电所中110kV母线三相短路故障
C．某区域电网中一座±500kV换流站双极闭锁
D．某地区一座4×300MW电厂，采用6回220kV线路并网，当其中一回线路出口处发生三相短路故障时，继电保护装置拒动
题型：多项选择题
阅读下面短文，用方框中所给单词的适当形式填空，使短文意思通顺、正确、连贯。
Everyone wants to have healthy teeth. When you laugh, you will open your mouth and _小题1:______ your teeth. The healthier your teeth are, the happier you feel. Why is that?
It’s because your teeth are important in many ways. You can take .___小题2:_____ of your teeth by doing like these:
Brush your teeth ______小题3:___ a day, after getting up and before going to bed. And brush all of your teeth, not just the ____小题4:____ ones. Spend at least three _____小题5:____ each time you brush. Be sure your toothbrush is _______小题6:__. You should use your toothbrush for three months and then ____小题7:____ it.
Clean your teeth with dental floss(牙线). It feels ___小题8:___ when you do it for the first time, but soon you’ll get used to do that. Brushing __小题9:___ your teeth healthy. You also need to care about what you eat and drink. Eat a lot of fruits and vegetables and drink water _____小题10:__of other drinks.
Do you want have white and healthy teeth? Please brush your teeth!