单项选择题

Questions 17 to 20 are based on the following passage. At the end of the passage, you will be given 20 seconds to answer the questions. Now, listen to the passage.
According to the professor, why is biology the most demanding of all sciences

A．Biology studies complex living systems.
B．Biology deals with controversial issues.
C．Biology requires knowledge of other sciences.
D．Biology cannot answer every question about lif
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单项选择题
What will John do on Saturday
A．Finish his paper.
B．Go to work.
C．Cook dinner for his cousin.
D．Go to the auditorium.
单项选择题
In this section, you will hear several news items. Listen to them carefully and then answer the questions that follow. Question 21 is based on the following news. At the end of the news item, you will be given 5 seconds to answer the question. Now, listen to the news.
A．A 6-year-old boy inside the balloon fell out during the flight.
B．The 6-year-old boy was alive after the balloon landed.
C．The police said that there was no one inside the balloon when it landed.
D．It was the boy who broke the balloon free of its moorings.
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