从方框里选择正确的句子补全对话

Tina: Hi, Dave.    1   

Dave: Oh, it was awful and boring. I didn't have fun that day.

Tina: What did you do?

Dave:    2    I saw a dolphin show there.

Tina:    3   

Dave: But there were so many people.

Tina:    4    

Dave: About 5,000 people.

Tina: Wow!  Too many.

Dave:    5    I couldn't see it clearly.

Tina: Oh, that's too bad.

为什么采用O型密封圈时要在槽内加挡圈？