根据上下文意思，用所给单词的恰当形式完成下列短文。

        Toby came back from England yesterday. Two months ago, Toby and his classmate____(enter) a robot competition for his school. More than one hundred  ____(school) around the country joined in it, and Toby's school____(win) the first prize. As captain of his team, Toby felt very proud. He said it was a wonderful experience. But the prize sounded more fantastic. It was a holiday in England. “When the plane took off, I still  ____(do) think it was real. It was like a dream." Toby said. Someone told Toby that the first prize of the robot competition next time____(be) a holiday in America. Toby hoped to enter it then.

-- I heard that there are a few seats left for tonight's show.

-- Really? I was under the impression ______ they were sold out a long time ago.

氯化钙溶液中含有少量盐酸杂质，在没有指示剂又不能增加其他杂质的情况下除去杂质，最好选用

下列操作中只发生物理变化的是(   )