1,C
根据开头的第一句“The first Monday in September is Labor Day in the United States.”
2,C
根据开头的第二句话“It marks the unofficial end of summer and a traditional signal that school is about to start”（翻译成汉语就是它标志着一种非正式的暑期的结束和是一种传统的标志着新学期将要开始的信号）
3,A
根据第二段中的这句“Communications Director Chyrl Hill Lander says the earlier start time makes it possible for students to take semester（学期） exams before winter vacation in December.”可以知道答案是A
4,A
应该是before
5,C
文章通篇都在说学校开学的时间,所以应该选择C