单项选择题

Questions 28 to 30 are based on the passage you have just heard.

A．They can easily catch fire.
B．They can be easily operated.
C．They are not as safe as gas balloons.
D．They cost too much money.

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单项选择题

填空题

将their改为its。

Questions 28 to 30 are based on the passage you have just heard. 您的浏览器不支持 audio 元素。