对于现代销售员来说，携带手机是必不可少的。

For a modern salesman, carrying a mobile phone is ___obligatory.

传统的丹麦午餐包括必不可少的配有鲱鱼的面包。

A traditional Danish lunch includes the ___obligatory bread with herring.

在许多正式场合，准时到达是必须的。

In many formal events, arriving on time is ___obligatory.