He is always late for his first _______ in the morning.
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I borrowed a book from the _______ yesterday.
The boy wants to take the _______ course next term.
If you had worked harder, you would have got an A in _______ Li’s course.
1、单片机应用系统常用的蜂鸣器（电磁式蜂鸣器）有哪两种，特点如何?
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