单项选择题

Passage One
Questions 26 to 28 are based on the passage you have just heard.

A．A controlled study.
B．A trial study.
C．An observational study.
D．A successful study.

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单项选择题

单项选择题

Passage One Questions 26 to 28 are based on the passage you have just heard. 您的浏览器不支持 audio 元素。