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Passage One Questions 11 to 13 are based on the passage you have just heard.

A．Much smaller than a human hair.
B．A little bigger than the bacteria.
C．One billion times smaller than crystals.
D．As big as a virus or a cell.

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Passage One Questions 11 to 13 are based on the passage you have just heard. 您的浏览器不支持 audio 元素。