单项选择题

Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions.
Now listen to the passage. According to the speaker, what is the problem with lignin

A. It is difficult to obtain.
B. Paper cannot be made without it.
C. It causes paper to deteriorate.
D. It prevents wood from being turned into pulp.

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单项选择题

单项选择题

Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. According to the speaker, what is the problem with lignin 您的浏览器不支持 audio 元素。