法律规定在汽车上必须系安全带。

Wearing seat belts in cars is ___compulsory by law.

在我学校，游泳是必修的。

Swimming was ___compulsory at my school.

穿着传统服装不是强制的，但鼓励这样做。

Wearing traditional dress is not ___compulsory, but it's encouraged.