Definition: If something is obligatory, you must do it because of a rule or duty. Level: C1
在一些国家，服兵役是强制性的。
Military service is ___obligatory in some countries.
这门课不是必修的，但很多学生都参加。
The class is not ___obligatory, but many students attend.
所有员工都必须穿制服。
Wearing a uniform is ___obligatory for all staff members.
