单项选择题

Questions 11-13 are based on the following passage about an introduction to balloons. You now have 15 seconds to read Questions 11-13.
What enabled the earliest balloons to fly

A．Gas produced by metal and coal.
B．Air kept hot by burning coal.
C．Hydrogen heated by a gas burner.
D．Natural gas carried in a metal container.

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单项选择题

单项选择题

Questions 11-13 are based on the following passage about an introduction to balloons. You now have 15 seconds to read Questions 11-13. What enabled the earliest balloons to fly 您的浏览器不支持 audio 元素。