Which day shows that a new school semester will begin in Ame

Which day shows that a new school semester will begin in America?
The first Monday in September is Labor Day in the United States. It marks the unofficial end of summer and a traditional signal that school is about to start.
But now public schools often begin sometime during August. They have different reasons for starting early.
The Tucson Unified School District in Arizona began classes on August eleventh. 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. She says the school district also wants to follow a calendar similar to that of the nearby University of Arizona.
Many schools in Virginia still open the day after Labor Day. But the public schools in Montgomery County, in the southwestern part of Virginia, began classes on August twentieth. Human Resources Director Mark Pashier says his county has been opening schools in August for at least the past eight years.
The main reason, he says, is the tests that schoolchildren in Virginia take every spring. School officials want the students to have the most time to prepare for them -- which means starting the year before Labor Day.
A September 1st
B The end of sum
C Labor Day
D The first Monday in September

A