Which among the following are the examples of mechanical energy reservoir?

Which among the following are the examples of mechanical energy reservoir?
(1) Furnace where fuel is continuously burning
(2) Raised weight
(3) Flywheel in running condition
(4) Compressed spring

a. (1), (2) and (4)
b. (1), (2) and (3)
c. (1), (3) and (4)
d. (2), (3) and (4)

Correct Answer : d. (2), (3) and (4)

Explanation :

A mechanical energy reservoir is a large body which can store work as potential energy. This large body is covered by an adiabatic wall. Among the above examples, furnace can not store work as potential energy. So furnace will not be the example of mechanical energy reservoirs. Raised weight, running flywheel and compressed spring can store the work as the potential energy.