In this section, you will hear several passages. Listen to the passages carefully and then answer the questions that follow. Questions 11 to 13 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. What is the main topic of the talk
A．The raising of silkworms. B．The weaving of silk. C．The invention of rayon. D．The strength of rayon threads.