介绍： 1. obligation: If you have an obligation to do something, it is your duty to do that thing.

Eg: When teachers assign homework, students usually feel an obligation to do it.

2. mortality: The mortality in a particular place or situation is the number of people who die.

Eg: The nation's infant mortality rate...

介绍： 1. obligation: If you have an obligation to do something, it is your duty to do that thing.

Eg: When teachers assign homework, students usually feel an obligation to do it.

2. mortality: The mortality in a particular place or situation is the number of people who die.

Eg: The nation's infant mortality rate has reached a record low.