Why did the writer say it was “the first day of school”?

A、Becausehewasanewteacher.;
B、Becauseitwasthefirstdayofschoolafterthewintervacation.;
C、Becauseitwasthefirstdayofschoolafterthesummervacation.;
D、Becauseitwashisfirstdayinthisnewuniversity.