obligatory

在大多数州，系安全带是法律规定的强制性要求。

Wearing a seatbelt is ___obligatory by law in most states.

街道上排列着用于可回收垃圾的强制性蓝色垃圾桶。

The street was lined with the ___obligatory blue bins used for recyclable trash.

游览巴黎时，在必不可少的埃菲尔铁塔前拍照是必须的。

When visiting Paris, taking a photo at the ___obligatory Eiffel Tower is a must.