Define
A. Ecosystem Services
B. Sere
C. BOD

Hint: The term ecosystem is generally associated with all the living, non-living and the environment in which they are living whereas a sere is a term used for a specific community and the BOD indicates about the oxygen levels of the water.

Complete answer:
There are various biological terms in which we can define the relationship between various biological factors.
A. Ecosystem Services
An ecosystem is a term which is used to show or tell the relationship between the living organisms, non-living organisms and the environment in which they are living together and are dependent on each other. The living components of the ecosystem are known as the biotic component and the nonliving components are called the abiotic components. It also tells about how these organisms are interacting with each other. The benefits which the humans receives by the functioning of ecosystem are known as ecosystem services.

B. Sere
The sere term is a type of species of an ecosystem which lies just below the climax community of a species meaning the community which is about to reach its climax community is often known as a sere.

C. BOD
The term BOD stands for the biological oxygen demand or we can say biochemical oxygen demand. This is generally observed in the water bodies. We all know that there are various organic compounds present in water bodies which need to be broken down. The responsibility of breaking down these compounds is done by the biotic components of the water body in the presence of oxygen. The oxygen can not be free in water just like the air and rather it is supplied to the biotic components in the form of BOD.

Note: The term BOD is also associated in determining the pollution levels inside the water bodies. When the BOD levels are low,the water is said to be polluted because then there will be larger organic compounds in water which need to be broken down but due to less BOD it can not be done.