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The source or starting point of something, such as a river, a problem, or a piece of information.

Very serious, stern, or severe. It is typically used to describe situations, challenges, or tests that are difficult to handle.

To use up or spend a lot of resources, such as time, money, or energy, often in a way that suggests a significant amount is required.

To keep safe from harm or injury. It can apply to people, the environment, cultural heritage, or rights.

The process of eroding or being eroded by natural agents; or the gradual destruction/diminution of something like rights or value.

To make something continue in the same way or at the same level as before. It often refers to order, living standards, or physical states.

Sources of power, such as fuel or electricity, used to provide light and heat or to work machines.

The state of being diverse; variety, especially in species or cultural backgrounds. It refers to the range of different things or people within a particular group or area.

To cause physical harm to something so as to impair its value or usefulness; or to cause injury to someone's health or reputation.

The introduction of harmful materials into the environment, such as air, water, or soil. It is a high-frequency topic in academic tests.