英语学习乐园:课程(在线收听

Can you explain the difference between the two phraces?
My favorite subject is history.
Proffesor Zhang's lecture is too difficult,I could hardly  keep up with it.
We have 8 courses this semester,including the ——.
required course必修课
elective选修课
subject科目
course课程
extracurricular课外的
keep up with跟上
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