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. If students don’t understand an important part of the lesson,______.A．they had better not to ask teachers to explain it B．teachers may be impatient with them C．they should ask other students for help D．teachers would gladly explain the things to them again
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